the best advice I have is to check out the stickies at the top of the AR section, read some of the build threads. After you've done that you will have a much firmer grasp as to what you want to look for.
Being so versatile and flexible it really depends on what you want to do with the AR more so than anything else. Iron out that detail, then start looking for rifles/parts that meet that criteria, the come back here and ask away
Have you given any thought on building your own?