I was an STP first timer, doing the 2 day version. I did the mass start at 0530 - it wasn't that bad. I've done local (Maryland) century rides with large (10,000+) crowds that were pure carnage at the start. It was nice when things started to thin out, though. I didn't stop at the first (19 mile) rest stop and that made a big difference in the crowding.
I planned on stopping at Chehalis, where my my wife would pick me and the rental bike up but was feeling so good that I pushed told her to pick me up at Winlock at mile 122. If I had it to do over again I would have booked a hotel room in Kelso and done 150 miles the first day and had mostly just the Rt. 30 slog left on Sunday.
The rider behavior was pretty much average for large rides I've done around the country - just as many faster riders wearing coordinated outfits on expensive bikes doing stoopid things as there were slower riders in street clothes on low end bikes doing so. Must have been a lot of people not used to riding on roads with bumps - on every steep downhill you could look ahead and tell a bump was coming by the scattered water bottles and energy bars...
I had a lot of fun - very well supported and planned out ride. I did a Cycleblaze journal on the ride - writeup
here.