The first two bikes that I ever bought were too big for me. I didn't know what I was doing when buying these bikes: I just wanted to get around while in college.
Ever since those two 'learning experiences' I've been far more careful about buying the proper sized bike.
For better or worse, I've used standover clearance as one easy deazy criterion for bike fit. I want at least 1" for a road, and at least 2" for off road.
I am back in the market for a road bike, and with a 30" inseam, I'm noticing certain brands I'm interested in don't offer much in the way of standover clearance.
The 52 cm specialized for example, has a standover of 30.3":
http://www.specialized.com/us/en/bikes/road/allez/allez-elite#geometry
With shoes, I should have a bit of clearance, but not a lot.
Do you all consider standover clearance an important factor when sizing a bike?