Freshman year, I tried to run a game that would go back to my roots. D&D 3.5, 1st level PCs, 4d6 drop and assign with utterly newbie players in the Sunless Citadel. I expected it to be a blast, and it almost was. The druid's animal companion was a bit strong, but other than that, things could have been OK. Except that I had one of Those Guys... a WoW veteran who went straight for the CharOp boards and ripped through supplements, and ended up running an area control Tome of Battle crusader with a spiked chain. My official policy on supplements at the time was "anything goes"; I had a fellow who wanted to play a warlock, and what's the danger of supplements for newbies, anyways? They were having enough trouble getting their heads wrapped around core that I wasn't concerned. Man, did that backfire.
So that game dissolved, and I started DMing for a group of established gamers. I guess I took away the wrong message; I thought it was just a problem with That Guy, and people like him. There was some cognitive dissonance going on, because I saw in him a mirroring of myself, and that led to my first post on powergaming. But in any case, when I went to run True20 for the veterans, I once again allowed supplements - the Warrior's and Adept's Handbooks (the Expert's Handbook was a terrible affront to True20's design ethos). And so we had one canny, sneaky thief... and an absolutely unstoppable warrior and a wizard who was immune to basically everything I could throw at him. That campaign died when I finally threw up my hands and went "Look, the only thing that can actually threaten Mr. Wizard here is a caster of like 6 higher level than you guys, using True20's Disjunction equivalent. There aren't many of those in this setting, and I don't have time to prep that ones that do exist. Also, if they come after you, they will bring a horde of orcs and you will all die." So we killed that campaign and converted to Traveller.
But during the post-mortem discussion for True20, we as a group realized that supplements were probably the real problem. The thief was skilled, and could sneak or lie his way out of anything, but was terrible in a straight-up fight. In short, he was what a thief should be. The warrior and the wizard were absolute monsters, and the pieces of their designs that enabled that power came primarily from supplements. So, for the Traveller campaign, we came to a consensus as a group to make things from supplements available only under the GM's purview, and introduced into the game in the manner of his choosing, rather than being driven by the players. And with that rule established, the Traveller campaign was pretty damn awesome. We've been running core-only Traveller and Trailblazer ever since, and those campaigns have likewise gone pretty well; none of them have folded over rules interpretations or inability to prepare for PC combat capabilities (though I think we're pushing the limits of Tim's willingness to prep in the current TB game... :\. )
My recent musings on playing with newbies brought to mind my own time as one, and how awesome it was. And I started wondering, "Wait... how did I get to be a terrible powergamer? I know I wasn't like this at the beginning." And I looked back through time. My first campaign was the 3.0 starter box. Went great, except for when my fighter died, but hey, that happens. Second campaign was right after we got the actual 3.0 books, and went fantastically; many dragons were slain, much treasure gotten, and much fun had by all. We had access to the A & B 3.0 supplements (Tome and Blood, Sword and Fist, and so forth), but we didn't take that much out of them. No prestige classes, and I don't think any feats, though probably some spells. After that... things started to go down hill. There was the 20th-level game where we went "Let's see what 20th-level characters can do... holy shit", the Midnight game with Thagg the feral half-orc half-dwarf giantblooded orc-tosser, the X-Crawl gestalt ubergenerics game, the Savage Species monster-gestalt game, our travesty of a Vampire the Masquerade game (featuring Olaf the Brujah lumberjack berserker), the True Sorcery thought experiments... urgh. Notable high points were several low-level games with my friends from high school (rather than my family), and my brother's Eberron game, which was distinctly more sane (though still quite high-powered).
And after a bit of reflection on this descent into insanity and powergaming, when I run ACKS over the summer, I don't think I'm going to be allowing stuff from the Player's Companion. Nothing against the ACKSPC, but... class and spell construction systems are really more GM tools, anyways.