I went to Yellowstone two years ago and am planning a trip this year. I had a great time! Here are a few links that you can spend a lot of time exploring:
Poison Ivy's post on ROT describing her trip in 2007
I really enjoyed her post and her pictures. She helped inspire me to go, and got me very enthused about my planning. I see that the link to her photos is broken, but maybe she can let us know if they've been moved. There have been other posts here on VFTT, which you can find with a search.
Yellowstone Net Discussion Forums
Yellowstone Chat Page
The Totally Yellowstone Chat Page
There are a lot of great posts here, including some incredible photos. The first two boards are very active, with great pictures posted all the time. These folks can answer many of your questions, although only a small percentage of the info. relates to the backcountry. Some of these folks are real hardcore Yellowstone enthusiasts, especially the dedicated wildlife watchers. If you're interested in wolves or bears, you will find a lot of information on the internet.
As others will agree, many of the popular tourist areas are sometimes very crowded. That being said, I think these classic American icons should be seen by everyone. There is nothing like getting stuck in a half-hour bison jam in the Hayden Valley. You can reduce your exposure to crowds with some planning. Try to visit popular spots early in the day, and remember that a few steps away from a paved parking area will have a huge impact. It is possible to view popular sights like Old Faithful or The Lower Falls in solitude, even at peak times. Do a little planning and walk a few steps from your car. If you venture a quarter mile down any trail, you will find almost no tourists.
Everyone has their favorite spots, but I enjoyed every place I went. I spent most of my time in the northeast corner, near the Lamar Valley. I really didn't do too many long hikes, focusing on some roadside areas and half-day hikes. I plan to do several full-day hikes this year.
PM me if you have specific questions. Have fun!