You can use a standing mixer to create dense foams, marshmallows, and other great modernist dishes. For modernist cooking it's imperative the stand mixer has an effective whisk attachment. You can also use a stand mixer for many traditional applications.
At first glance most stand mixers look fairly similar, but there are actually quite a few differences when you take a closer look. When searching for a good stand mixer here are some things you should consider.
Look for a mixer with beaters that don't have the solid center post since they are harder to clean. Multiple speeds are important but more than a handful is probably overkill. It is nice to have a really slow speed so you can start the mixer without blowing or splattering ingredients all over your kitchen.
When selecting a stand mixer you need think about how you will be using it for. If you enjoy baking then you'll need a unit with a dough hook and enough power to mix heavy cookie dough. The quantity that your are normally cooking for may also impact the size of mixing bowl and motor you require.
Finally, what accessories might you want to use down the road? Will you have use for a food grinder, pasta roller, ice cream maker, fruit and vegetable strainer, or sausage stuffer?