My purchase was based on picture/sound and then
Price
Resolution
Inputs
Inputs are very important to me.
I have a lot of AV sources, with PS3 and HD Satellite, I use Component leads for that (only one input, so I swap leads a lot). Computer connects through DVI socket, and there's a spare SCART on the back as well.
If you want to make the most of your new HD TV, then you'll need to figure out what you'll be watching. I couldn't be bothered getting a built in freeview decoder, as I knew I'd be using HD AV sources.
I went for a TV with 1050 vertical lines, as I use it as a monitor now and again, and even though I could have a got a larger TV with 720 lines, I wanted the extra resolution rather than screen size.
I'm looking to get another TV soon, but I think I'll hold out for a cheaper 1080p TV first.