1) Physically and emotionally, the men will have changed much more than the women. This is mostly physiology -- boys mature later, and are the ones who go bald. Plus, if they're very, very lucky, the men will also meet someone who can dress them better than when they were in high school.
2) If you have children, you will save yourself and everyone else a lot of time if you laminate some picture(s) of your offspring and staple them to your forehead.
3) That person you had a crush on in tenth grade? They're still going to look good twenty years later.
4) Someone will be out of the closet -- with a 50% chance that that person was in your homecoming court (note to Generation Y: this will be reversed for all y'all -- someone who came out in high school will be in a heterosexual marriage, with two kids and a house in Schenectady).
5) WARNING: you will drink more at these functions than you probably should.
6) There will always be at least one woman who has given birth to many children in recent years but look like they could do a guest-hosting stint on E!'s Wild On series.
7) At any point during the reunion, you will observe a large number of women congregating near the bathroom, whispering to each other and giggling every five seconds.
8) Someone's going to bring their high school yearbook.
9) The food will leave something to be desired.
10) Unless he or she attended your high school, under no circumstances should you subject your spouse to this function.