Bells & whistles, inputs & outputs, power...
That's the differences. Power really isn't that important as long as what it provides is enough to make you happy then it gives enough power - and most do.
Bells & whistles are nice, but I don't play with my surround settings at all really, so I would be happy with about 5 settings instead of 65. Likewise, you may feel the same and have no problem doing manual setup instead of putting a mic in your room to do the work for you. It takes longer, but saves you $$$$.
The thing that may kill the deal is inputs and outputs. If you don't have enough for the equipment you own, or will own before you want to upgrade, then you are stuck. But, most receivers can usually cover the important stuff you may need. Component switching I think is where receivers really lack. I think most decent ones should come with 4 inputs, yet they come with 3, and almost all others come with 2. 2 gives you HD cable or satelite plus a DVD player. That leaves your game system stuck on S-Video or composite unless the TV has more than 1 component input... which many do fortunately.
Lots of lesser expensive stuff is a great way to go... and get hooked.... and buy bigger - BIGGER - BIGGER!!!!