There's a lot of discussion in this thread (and others) about how "popular" Star Wars is when compared to other franchises, especially Avatar and Harry Potter. It's hard to make an "apples to apples" comparison, but I finally think I've run across a fair way to do it. I got the idea from a movie (whose name escapes me right now) - just compare how many copies of the movie my local library system owns. Libraries are pretty good about listening to requests, and library usage (as opposed to actual sales) tells you what's currently circulating / popular, rather than what people may have bought years ago but no longer really watch. FYI, this is the city of Columbus, Ohio, with a population of over a million people, so I think it's a pretty big sample size.
For ease of comparison, I added up all the different version (Blu-Ray, DVD, extended editions, etc.)
Avatar: 39
Harry Potter #1: 64
Harry Potter #2: 62
Harry Potter #3: 60
Harry Potter #4: 74
Harry Potter #5: 72
Harry Potter #6: 62
Harry Potter #7A: 75
Harry Potter #7B: 97
Harry Potter total: 566
Star Wars
Episode 1: 35
Episode 2: 39
Episode 3: 52
Episode 4: 46
Episode 5: 47
Episode 6: 43
Episodes 1-3 boxed set: 15
Episodes 4-6 box set: 14
All 6 episodes: 5
Lego Star Wars (all combined): 104
Clone Wars (all): 173
Star Wars total: 573
So, in summary... Avatar is just one movie. Sure, more are due out, and it may turn into a big franchise. But right now, it's just one movie, and both Harry Potter are almost 15 times more popular than Avatar.