I didn't get into DC until I read an issue of Superman that Byrne wrote and Mike Mignola drew and then the Invasion crossover is what hooked me into the rest of the DCU.
As for Steve Rogers vs. John Walker,the first time they fought was when Walker was Super-Patriot a bit before #332(I think #327 but not positive) but as Captain vs. Captain America,yes #350 was the showdown between the two.Red Skull became major bad-ass and the issue also had a John Byrne drawn back-up explaining how the Skull ended with a cloned body of Steve Rogers.
In hindsight & having re-read the Omnibus,yes the Evolutionary War doesn't hold up that great but for me,all the characters were new and exciting and I had no clue who people were so I still hold it fondly as a story.Yep the What if? issue was better than the whole original story and I dug it immensely.
Red/Silver armor Iron Man is my fave armor hands-down.
I did miss the Mutant Massacre
I don't believe Groo was a back-up and his run in Marvel's Epic line was awesomeness. MULCH!!
