Love Mathmom's dresses - I would totally wear them!
OP, I say don't worry about WHAT to wear, just think about WHO is wearing it - YOU! Wear something that is very you - something you are comfortable in, something you look good in - and if you do have a "style" - go buy something new so you feel fresh and great in it!
Khaki pants would never be me. Black pants would never be me. But some skinny jeans with nice shoes and a cute top and a light cardigan WOULD be me! Or one of those Modcloth dresses a few posts up. A long to the floor casual dress would never be me, but a colorful sundress would!
The point is, there is no "uniform" that is appropriate - IMO, a reunion is a time/opportunity to show who you are through your dress. If you are not into clothes that much, then wear something comfortable but go get your hair done just before the reunion. Embrace yourself!!!