I guess to start you off...
- Budget?
- Slimline/Full?
- Integrated/Freestanding?
That should thin out the results nicely.
You may want a manufacturer that's got a longer warranty?
After that, depends on your usage. Some dishwashers can have a variety of different wash programmes, but if you're used to just sticking it on the same programme each time, then there's little point going with one that offers 15 wash programmes.
I would also consider what sort of things you load, as some have different basket layouts that can accomodate larger pans/dishes etc.