1. Did not create the character.
2. Capt. America’s creators were screwed out of their royalties by then publisher, and future Marvel publisher Martin Goodman. Stan Lee was his wife’s nephew.
3. Stan Lee ratted Simon and Kirby out to his bosses after Simon and Kirby decided to start working on material for DC. They were fired and Stan essentially took over Joe Simon’s job running Goodmans comic business.
4. Stan Lee allegedly did not back Kirby up to his bosses when Kirby wanted a raise because he was co-plotting books as well as the fact Marvel owed him money for helping in their Simon/Capt. America lawsuit. Kirby wanted to be recognized with Lee and share in the spoils of their work. That did not happen.
5. According to Alan Moore, Stan Lee has slipped up a few times and said that ‘he created Captain America with Kirby.’ Which is untrue.
Much as I will always love Stan, it’s important that he gets called out on his bullshit (and the bullshit by others—including fans—done in his and Marvel’s name), especially that same bullshit is currently burying the legacy of Jack Kirby.
I’m not going to get into the pros & cons of this argument (to be fair, Lee’s first published credit was a Captain America text story way back when), but this is important to also note. This is from an interview with Kirby in Comics Feature #44, May 1986:
Although both (Lee & Kirby) live in the Los Angeles area, they haven’t seen one another in a very long time, (however) Kirby’s attorney did have direct contact with a film project which Lee is involved with. When Cannon Films began advertising their forthcoming Captain America film, Kirby found he could not remain silent.
"They’d advertised that Stan Lee created Captain America. He wasn’t even at Timely then. We had to put a stop to that! We got Stan Lee’s name taken off the film as the creator."
This post at Kirby Dynamics is also worth a read; it features scans of a 70’s-era article in which Lee details how he supposedly came up with Spider-Man from whole cloth; note that Lee barely mentions Kirby or Steve Ditko. (This creation myth was also repeated virtually verbatim by Lee during his Oz Comic-Con Q&A a few months ago.)