I have to admit something that nobody else has posted here yet...my main reason for not going was financial! I'd always planned to go to my 20th, since I'd gone to my 5th but missed my 10th. I had fun in high school, and enjoyed my time there. When my 20th rolled around, though, the invitation said it cost $100 per guest (at a crappy hotel near the airport)! I'd also have had to book a hotel for the night, since it was several hours away from my home. So, between tickets for my boyfriend (who hadn't gone to my school) and myself, plus hotel room and gas, it would have cost me at least $400-500 to attend.
I checked the guest list and there were people I was friendly with, but nobody I was all that close to or dying to see (the ones I was really good friends with, I'd either stayed in touch with or gotten back in touch with already)...so I decided against it! You've just got to weigh all the options and then see what your gut tells you--then you'll find making a decision much easier!