Federer
Like: effortless brilliancy, elegance, fluidity, mix of classic and modern motion, 1 hbh, small racket, variety of shots, the dare slice, the way he sees the ball through contact, his appreciation of tennis history, very good volleyer by today's standards, the ability to raise the level of his game when facing break points and on tiebreaks, great understanding of the game and opponents flaws, great anticipation, he can read poor technique players -Roddick- like a book.
Dislike: stubbornness to work in improving aspects of his game like backhand, more aggressive return game, non use of drop shot, net game has declined over the years. Maybe he doesn't want to risk messing things up, but I wished he tried harder earlier to adapt to several threats like Nadals high topspin fh to his bh maybe by trying a more extreme grip for certain shots. The way he seems to loose concentration almost get bored often en 2nd sets after dominating the 1st. The handbag, the blazers, the perfumes and all that crap is very annoying. His arrogance when he says, "I played great", etc. I wish he was a little bit more modest. I just think he should let other people comment on his brilliancy.
Nadal
Like: mental fortitude, ability to hit winners from awkward positions, insane topspin, never give up attitude. Drive to continue improving and adapting to new conditions (grass, hard courts). Great physical endurance, speed.
Dislike: annoying time wasting routines, excessive bouncing, butt picking, often interrupting opponents' serve, delaying at the start the match by making the umpire and opponent wait at the net while he arranges his bottles and then sprinting right after, ridiculous outfits, excessive shot celebrations. Ugly tennis (but very effective), very consuming, hate the reverse forehand as a standard fh, condescending humbleness at interviews.