A good balance between all the elements. Speed, laterals, airtime, etc., etc., etc. I love coasters that do one thing and do it well like MF (speed) and ST (airtime) but one of my all time favorites is Viper at SFGAm because it has it all.
------------- "Seek not outside yourself." - ZeroOne
Jeff said: "Personality, lots of curves and a thong."
Damnit, Jeff! You beat me to it. I'm always looking for that package in a coaster.
Getting back to the matter at hand, though. (Hmm... as if curves aren't always the matter at hand.) I get a big kick out of airtime, but it's a mix of things that gets me going. I love long, steep drops with nice positive vertical G's at the bottom. I really get turned off by too much lateral power, since I'm normally the one who gets crushed.
No mention yet of visuals, flying past things so close yout think you're gonna lose something. Visuals also increase the sensation of speed and darkness reduces your vision also increasing the sensation of speed by screwing up your depth perception. I also like room to float and lack of arm restriction in rolling stock.