Having delved into a general recommended reading list elsewhere, here's my overview of the Gruenwald years.
When Gruenwald assumed control of the series, the Red Skull was dead (from issue #300) and he felt Cap lacked a strong rogue's gallery. Attempting to bolster up the ranks of villains, Gruenwald looked to the world around him to see what threats menaced the USA then. His first villain, Madcap (#307), embodied the USA's own disaffected youth; his most frequently-used villains the Serpent Society (#310) were a take on the evils of unions (ie, what if the super villains unionized?); finally, Flag-Smasher (#312) was an anti-nationalist, a man who rejected everything Cap believed in.
Flag-Smasher works well as a zealot, someone whose political (or anti-political?) convictions were firm and honest, but whose means (the violent overthrow of all nations to establish a one world government) put him squarely in the villain camp.
Gruenwald sent a surprising number of villains to their deaths in his first year and a half on the series. In #315, he dispatched the Porcupine, giving the somewhat-goofy villain a sad end; the Porcupine wants out of crime and hopes to make some quick cash by selling his equipment, but no one in the underworld is interested. When Cap finds out the Porcupine has a lead on the Serpent Society, he offers to buy the Porcupine gear if he'll help trap the Serpents. It all ends badly for the Porcupine, but Cap is true to his word.
The greatest massacre of villains went down in #319-320; for the previous year (mostly in titles which Gruenwald edited), a figure would step out of a crowd, shoot a super villain in the chest and declare, "Justice is served." In Cap#319, we learn this figure is the Scourge of the Underworld, who massacres an entire room full of criminals. Cap tries to catch the killer in #320, but is denied any real satisfaction; the Scourge's explanation for his behaviour is almost immediately discredited and he escapes capture...the hard way.
In a rematch with Flag-Smasher in #321-322, Cap is forced to take the life of one of Flag-Smasher's operatives. This caused a major stir in the series - one which actually lasted for decades! In these issues, Cap states plainly that he doesn't kill. Many fans objected to this interpretation of Cap, declaring he "surely" killed people during World War II. Gruenwald stuck by his guns and his interpretation remained with the character until the advent of Ed Brubaker in 2004.
Issues #323 & #327 introduced John Walker, the Super-Patriot, another choice antagonist for Cap. The Super-Patriot was a gung-ho, Oliver North-meets-Ronald Reagan-meets-Bruce Springsteen-meets-New Coke 1980s American. At least, that's what he seemed to be on the surface. While draping himself in the flag to promote himself like a rock star for public appearances, the Super-Patriot would hire his own enemies to beat up for the cameras! Although Walker claimed to be every bit the patriot Captain America was, it was a false patriotism, nothing but jingoism. Unfortunately for Cap, the Super-Patriot was his superior in physical power; he fought Cap to a standstill while barely trying, with Cap unable to find an advantage. Surprisingly, Walker was being set up to become a fascinating hero in his own right.
In #332, the Commission on Superhuman Activities - a body of US government officials - having realized Captain America is the same Steve Rogers who entered government service in 1981, demand he resume working directly under their supervision. In this, the era of the Iran-Contra scandal, Steve isn't certain he can surrender his own destiny...so he quits. The government owns the Captain America costume and shield, but by the end of #332 that's all they have.
In #333, the Commission hires John Walker to become the new Captain America and he employs one his hirelings to be his Bucky (later taking the less-insulting name Battlestar). For a time, the series splits between following the new Cap & Battlestar as they learn how to fight and begin taking on missions for the government, while Steve adopts his new identity ("the Captain") and joins his ex-sidekicks in fighting crime with much-reduced resources.
The storyline hits a major turning point after Walker's secret identity is made public by two of his ex-hirelings (now dubbing themselves Left-Winger & Right-Winger). By then, Walker had already made enemies for himself and in #345 one group, the fundamentalist Watchdogs, took Walker's parents hostage to trap him; Walker tried to save his parents, but failed. At seeing his parents' death, Walker's mind snapped and he killed and maimed several watchdogs, then settled down with his parents' bodies, chatting to them as though they were still alive.
In #347, Walker went after Left-Winger & Right-Winger, leaving the duo in a death-trap (more on that in #383). He was clearly coming unglued and matters finally came to a head in #350 when the Red Skull returned, revealed as having been manipulating the Commission from behind-the-scenes. The Skull tricks Walker and Steve into a fight, but this time Steve proves himself superior.
The Commission gives up trying to control Captain America, having learned their lesson; they offer to let Steve have his costume & shield back, but Steve's learned he doesn't need them to be a hero. It's only when Walker confronts Steve directly about how he carries the identity better than anyone that Steve relents; it's a great moment for Walker, having learned his lesson about who Cap is. Walker and Steve's relationship going forward has remained an interesting one, because Walker is one of the only people who can understand what it means to be Captain America...but he still doesn't see eye-to-eye with Steve and won't hesitate to disagree with him. He's found his own path as the U.S. Agent, usually in the government's employ.
The Bloodstone Hunt (#357-362) was just good fun. The Serpent Society's Diamondback had been an unusual love interest for Cap: he hadn't had to deal with a criminal falling for him before. In this globe-spanning epic, Cap works with Diamondback to stop Baron Zemo and Batroc's Brigade from reassembling the Bloodstone, an alien gem of vast power. Diamondback keeps trying to prove herself to Cap and this ultimately sets her up to reform for good. In addition to the many Indiana Jones-style locales and death-traps, Batroc brings some fine levity to the proceedings, particularly in the finale when he has to point out to his own men how unlikely it is they'd win in a fight with Cap.
The "Acts of Vengeance" crossover set up most of Marvel's top super villains to collaborate against the heroes. Consequently, the X-Men's top villain - Magneto - was working alongside Cap's top villain - the Red Skull. In #367, Magneto finally confronts the Skull; being a Holocaust survivor, Magneto has just one question: is the Red Skull the same man from World War II? Once Magneto has his answer, he goes into full-on revenge mode and...hey, it's the Red Skull...you never feel sorry for him.
Cap and Diamondback formally began a relationship in #371, despite Cap's many protestations that he couldn't make a romance work (Diamondback proposes "friendship" instead, then asks her "friend" what he's doing that evening). Gruenwald had done away with Cap's attempted secret identity around #318, having him be Captain America full-time. Here, Steve is forced back into the real world and finds he's really out of touch. While he and Diamondback try to have a peaceful night, Diamondback's friends hang on the outskirts to make sure no one interrupts the evening. A funny and welcome change of pace for the series.
Gruenwald's next great epic was "Streets of Poison" in #372-378, drawn by my favourite Cap artist, Ron Lim. Cap goes up against the drug trade, putting him in the path of the Kingpin and Bullseye...plus the Red Skull for good measure. Unfortunately, Cap winds up being exposed to drugs and they bond to his own Super-Soldier formula, meaning his body can't expel their effects. Cap goes completely off the deep end, leaving it up to Diamondback to reel him back in. Because of the nature of Cap's origin - particularly as Kirby first depicted it in 1941 - it had often been noted how Cap owed his powers to drugs, drugs which were essentially "super steroids." It proved to be a strong jumping-off point for Gruenwald to tackle and ended with Cap losing the formula, helping Gruenwald make his point that Cap is exceptional even without his powers (just as in the Walker epic he showed Cap didn't need his costume or shield).
In the midst of celebrating Cap's 50th birthday in #383, Gruenwald included a back-up tale where the U.S. Agent goes looking for Left-Winger & Right-Winger...only to find them six feet under. In horrible, graphic detail, we learn how the death-trap Walker left his ex-friends in didn't kill them...but left them so mutiliated that they took their own lives. It's a great turning point for Walker as he confronts his personal demons and vows to become better.
I never grew bored with Gruenwald's Cap, but the years following Lim's departure weren't quite as interesting. Still, as a fan of the Cap-Diamondback romance, there was always that developing relationship. At one point, Diamondback had been almost drowned to death and wound up tracking down her assailant and drowning her in return. After keeping this murder a secret from Cap for more than a year, in #424 Diamondback finally confessed it all to him; Cap's reaction? Unconditional support, whatever happens. I love ya, Cap.
Gruenwald brought his run to a close in #443. Cap eventually regained his Super-Soldier formula, but the drug was now working against his body, causing him to become weaker the more he exerted himself. Of course, this is Cap we're talking about...he refused to retire, instead pushing himself closer to his grave. In this, Gruenwald's final issue, Steve is told he has one day to live. He tries to find a friend to spend it with, but no one seems available. Ultimately, Cap winds up having a heart-to-heart with Batroc, sharing all of his trauma with one of his enemies. Cap even asks Batroc to reform, which I wish had happened.
Mark Gruenwald died only a year after leaving Captain America. I wonder if he'd have enjoyed Captain America: the First Avenger? He'd surely have been pleased to see Cap on the big screen, finally receiving some dues from the world at large.