There's a little more adjustability available with riser bars (check out
http://www.surlybikes.com/spew2.html for details) but for the most part there aren't any real differences. Riser bars just happen to be trendy right now. The biggest difference I've seen is that most flat bars don't have much sweep (the angle that the end of the bars bends towards you). I run Salsa Pro Moto flat bars that have 11 degrees of sweep (I think most riser bars are only 8 degrees and flat bars are typically 3 to 5 degrees) which I find very comfy on my wrists. The Surly Torsion Bar has 15 degrees of sweep and is cheap (and heavy) but the extreme sweep requires a longer stem to keep the same reach to the bars.