Size matters. You want one to be the right size, if you are usually cooking for one or two, you won't want a huge one, and if you are usually making vats of food, a small one would be useless.
My dream crockpot, which I don't believe exhists (but I will say I have not checked for a while)
- removable cooking vessel, and all removable parts dish washer safe
- Timer to shut it off (going to warm when it's done)
- Timer to turn it on
- Auto shift, where it cooks on high for an hour and a half to heat it up fast and shifts to low on it's own
- see through cover
- Liner can go from the fridge to the cooker, so you can set it up the night before and it won't crack when it's heated
One thing I highly recommend is crock pot liners, you can get them in the grocery store for just under $6 for a pack of four, or you can order online in bulk and get a box of 240 for $39, and everything in between. Makes cleaning up so much easier.
We have a couple, no brand affinity just what was on sale at the time. Our large one is a Crock Pot brand oval with an electronic timer (if you are prone to power outages don't get an electronic timer, it will shut off and not come back on, even if the outage is just a few seconds).
We have a smaller Proctor Silex that has a knob that says off, high, and low, no timer, no electronics, etc.
Think about what you will be cooking, if it's mainly sauce, chili, stews, round is fine, if it's things like whole cuts of meat, chickens, turkey breast, etc you may want an oval one, the larger cuts fit better in the oval one.