For me it's easy: the ability to wrong foot opponents It's something I always try to do against the computer but only works occasionly (though still miles ahead of ts3) Anyway I just wanted to rave about how much I enjoy online matches and the thing I didn't feel so much in ts3 - the insane mental game involved. I almost felt in ts3 even if you guessed the right way, you'd still lose the point 80% of the time anyway... but in ts4 it's so thrilling when you win a point because you felt you actually out thought the opponent, and hit it back to that one corner just one more time than they thought you would. Also even on the serve I actually have to think out whether I want to belt one down the T or slice them out-wide, because it can make such a huge difference. Overall, I just absolutely love the aspect of having to anticipate how the opponent will anticipate your shot. This is easily the best game I've ever played, tennis or otherwise.
p.s. Also I'll likely be playing tomorrow night around 16:30-19:00 ish (GMT-6) if anyone wants to arrange a match, I'd love to, just add me - dahltennis - and send a message. I'll be playing as Petra Kvitova (who I did a pretty good job making in my opinion, but you be the judge)