What to look for in a Stand Mixer?
I've got one of the professional series Kitchen Aids (and my parents have the traditional version to compare and contrast).  If money isn't a huge deal, then getting the professional series comes with a more powerful motor and a heavier base that will stop the mixer from "walking" around the counter if you put dough or something dense in the mixer.  

One odd thing that I didn't expect is that the mixer is really inconvenient for small things.  It can whip up like a triple batch of whipped potatoes, but it has the darnest time whipping up a little bit of whip cream or a little merangue.

Kitchen Aide's attachments are pretty rock solid... One of our more "fun" attachments that we have is the ice cream bowl.  Freeze the bowl, dump the ingredients and set the paddle on low.  Again, with the professional model, the motor is strong enough to churn a bit longer than traditional other ice cream churns...

IMO, if you're a big dough cook, go w/ the stronger model, otherwise the smaller models should be great.

EDIT
In an interesting note, my sister has one of the smaller models and it did infact "walk" off the counter a few weeks ago... The whole machine went crashing to the ground... She picked it up, dusted it off, but the only damage was a small piece of plastic broke off... Gotta give it to Kitchen Aide