To give the same quasi-copypasta spiel I've been giving for a while (in that I just typed it but I've been saying it over and over again):
Haruhi is a gung-ho son of a bitch who kicks reason to the curb to get his club. ...then he gets all mopey bishie when things don't go his way.
Yuki is the cool silent type. He's just calmly surveying the situation but when the shit hits the fan he will whip out his MySQL and kick your ass.
Mikuru-kun is a pure uke. He moans when Haruhi forces him into various outfits. There's really no nice way of putting this one: you are gay for liking Mikuru-kun. But so deliciously gay.
Itsuko is a slut. Just smiling all the time, saying "Oh, are we molesting Mikuru today?" and being a total dyke for Kyonko.
Kyonko is sort of like a Haruhi Fujioka or whatever. Kyon's exasperation and dry wit becomes really sexy when a girl does it. But of course, deep down she's having the time of her life. Also, her ponytail is moe.