A lot of stuff to cover here, but i will give you a little info. In terms of differences b/w companies, it is just like cars, the sunfire and cavalier are very similar, but the differences are cosmetics, and the some of the parts are different. From these changes the sound, power supply....etc, is slightly different b/w receivers. That is why when it comes down to it, the choice between receivers is personal preferance, the reason to come to this site is to mainly find out if you should avoid a certain receiver due to problems that they are having.
In terms or 5.1, 6.1, 7.1, some manufacturers still make 5.1 and they are a lot better than some of other 7.1 setups. What they are doing is trying to future proof for when there is the release of dvds in 7.1 setups. Also, when you go from a 6.1 to a 7.1 and move on up the line in the same manufacturer, then you are adding better components and dac's to make a better sounding receiver as well as more channels.
hope this helps a bit, and hopefully someone will chip in with more info.
