Obviously, there's the amount that it can make in one batch, and there seem to be three basic types (ones you freeze the bowl ahead of time; ones you pack it with salt and ice, and ones that refrigerates themselves).  ... and I know I'm looking for one that I don't have to hand-crank ... (I did more than enough of that as a kid)  ... but beyond that, are there any features I should be looking for when buying a machine?  If it makes a difference ... I'd mostly be making sorbet with it, as I don't handle dairy well, but I might make ice cream or gelato to share with other people. 
I have a Donvier brand ice cream maker.  It's a hand-cranked kind with an insert that you pre-freeze.  Unlike the old-fashioned salt/ice kind, the hand-cranking involved is super minimal, so don't necessarily discount that option.  (Basically, you just need to scrape the ice cream off the sides of the insert occasionally, so it's usually something like crank it around 2-3 times every 5 minutes and it only takes about 20 minutes to freeze the ice cream.... you'll likely spend more time stirring your ice cream/sorbet mixture during prep, than you will after adding it to the machine!)  I would personally recommend this style since it means there are fewer components to break and they tend to be much less expensive.  The pre-frozen container is part of what makes it so speedy, but obviously it means you can only make 1 batch at a time and it takes up space in your freezer.  I think the only reason to get the kind with built in refrigeration is if you want to make multiple batches in a row and you plan to use it a lot.  Whichever style you choose, buy the largest one that your kitchen can accommodate since you can make a small batch in a big machine.  I got mine at a garage sale and as a result, it's the pint size, rather than the quart size and it's the one thing I don't like about it. 