Timer switch is handy - I wish mine had one.
A big capacity, so you can make double and freeze some (I do this all the time!)
Two heat levels (most have this)
Think bowl to evenly distribute the heat.
Sturdy lid - I've heard that fragile ones can break easily (esp if you are clumsy like me!)
Also make sure the bowl can fit in your fridge as I often assemble the night before, leave in fridge overnight and then cook in the morning.

I use my slow cooker weekly at least and my hubby loves anything I make in the slow cooker!

PS. Shop around - mine was $30 cheaper at Myer than Target at the time!