I started playing trem at the beginning of march this year. I read all of the tutorials, and played humans for a month before trying out alien. I am fairly proficient at most weapons/classes. I have recently started learning flamer, md, and luci on humans, but have not started rant, and have limited basi experience. I am good at building human and alien. I found the game fairly easy to learn (ofc, I had played no FPSs previously other than doom on dosbox :P).
I would have to say that the hardest bit to learn for alien was dretch. Next to that was learning how to use goon and mara after getting into the dretch mentality. For humans, I'd say the hardest to learn was how to track aliens. I think the game is fairly easy to learn if someone sets their mind to it. I have put 308 hours, 40 minutes into this game, and have become fairly good.
I think that what new players really need is ready access to tutorials, since this is what helped me the most. This is not the same as posting them on the forums. Many gamers do not seek out tutorials (I have come to believe that I am an anomaly in this sense), and will only read short bits, and only if prompted (especially veterans of other FPSs, who "know" how to play already. I suggest a series of 10 basic tips to be presented on first startup, with a link to more tutorials which could be accessed in-game. Some videos might be helpful as well for visual people, as well as interactive ones for 'hands-on' learners, all accessible in-game.
Anyway, my 2 cents :P