I was watching a video recently and one of the things that kept popping up was the huge difficulty curve on Red Orchestra 2 (haven't played it personally) compared to a lot of other games.
That got me thinking, do you consider TF2 a game with an easy or hard difficulty curve?
My first instinct was that it has an easy curve. I remember when I first started playing, everything was easily recognizable and identifiable and I don't recall any particular aspect of the game being hard to figure out.
There could be finer details that are much harder to learn (Rocket jumping for newbies, playing Spy)
BUT, I noticed after TF2 went Free to play that some people I know to be FPS players didn't really catch on with TF2 very well. They played it a lot then just stopped completely, which leads me to wonder if its more difficult than I realize or if them being console gamers (gamepad vs. keyboard) hurt.