Heroes Unlimited: I picked up the book with the Steranko art. The classes looked interesting. The combat system looked decent with cool things like Roll w/ the Blow.
Then, I went to make a character. Palladium should have been sued for false advertising with the title. It is one of the most limiting superhero games I have encountered (or perhaps the most). On top of that, many of the mechanics that I disliked (for a superhero game) exposed me to the very things that I think make it necessary to go an M&M route to make a good d20/OGL superhero game.
Hero System I love the character generation. However, I hate looking at a Champions character sheet and combat, in my experience, bogged down very fast. Also, I didn't care using it for magic in a fantasy game.
Iron Heroes: The premise for the game and my like of the Book of Iron Might is what attracted me. However, the classes, the skill groups, tokens and other mechancis just turned me completely off from the game. Hmm. 4e is almost an exact repeat and I doubt that I will look at another game by Mr. Mearls as lead developer/mechanical developer.
Star Wars d6: Star Wars it has to be cool, right? The mechanics just left me cold, but I still like it better than the d20 versions.
Warhammer 1e: I love the setting. The basics of the combat mechanics.
Superworld, Elric!, Call of Cthulu All of these games seemed really cool. However, each time got to combat, the basics of resolution (how defense and parry worked), just turned us off. Maybe we were handling defense and parry incorrectly, but we just gave up.
Marvel Superheroes Roleplaying Game: Not truly failed first contact, because I houseruled the hell out of it and played it until discovering other superhero games. However, it would have failed if I had known of other alternatives for superheroes.
I loved the Marvel Universe of the 1970s and early 80's and an rpg sounded great. Unfortuantely, the game did not failed to capture the feel of Marvel comics for me. Spiderman can't catch a cold. Combat and Reputation mechancis led to situations you won't see or rarely see. The ranks were full of numbers that meant nothing when what it needed was a few additional ranks and revised strength ratings. Oh, and many charactres were poorly researched.
Marvel Superheroes Adventure Game: Some noticeable improvements from the MSHRPG mechanics (e.g, actual opposed checks and allowing more variety of ability scores). Unfortuantely, the combat mechanics still resulted in situations you won't find in the comics or were much more common. Plus, the movement rules horrible. It really took Steve Kenson (of Mutants and Masterminds) and the rest of the MSHAG community to rework the game into something worth playing, imo.
And, again poorly researched characters. One of the sourcebook writers admitted on a message board that he was not familiar with most of the characters in the book he wrote. He was really only familiar with one title and interested in another portion of the Marvel Universe.
DC Universe Roleplaying Game I checked out the book on a recommendation and it looked spectacular (except for the newsprint paper). Everything looked good until I got to lifting, movement, and the manipulation powers. Lifting and movement were tied to huge charts of numbers that relied on the die code for column and a skill roll to get results making it difficult to pin a rating - huge jumps in scale and way too many possible results for my taste. The revised lifting and movement rules in the power supplement did not fix the problem.
The manipulation powers also suffered from being tied to die codes, because you had to have die codes to gain access to specific abilities. Oh, wait, you didn't! You could trade things around, but there was no real guidelines and the structure got messy.
AD&D 1e and 2e (these days)
Rolemaster was an interesting case. My friends, who only had played DND, initially found it too complex. However, they later gave it another attempt when a new addition to the group offered to run it (by this time, they had exposure to other games). Rolemaster became their favorite fantasy game. Unfortunately, we had to stop playing it, because two players moved cross country and the new additions to the group had problems with the math.