What do you get for your money when you spend more? HD, IS and silly zoom levels are fairly cheap nowadays, but some are still a grand. Is it just image quality or what?

Sensor quality.
Just because something records in HD doesn’t mean it’ll be good quality.

I spent the extra to get a 3MOS camcorder, as they’re far better in low light. I’ll probably replace it next year though, as technology has moved on a bit in the past couple of years.