Not sure if being overtaken by people in fancy dress has been mentioned yet - not too bad if it something not restricting them, but a someone with a 4 foot tall Giraffee head overtook me once.
Also, I've seen quite a few people with 'Run Easy' or 'Run at the speed of chat' t-shirts - that's a bit of a downer when someone who is apparently 'taking their time' is passing whilst your busting a gut.
Last year one of my clubmates put just under 2 hours so 1:55 or 1:58 or something and was in the second wave. The rest of us who put 1:50 or less were in the first wave. They are also split into colours - so one clubmate who put sub 1:30 was in 'White' and basically up with the Elite, then there was another pen for 1:30 - 1:50 with the rest in the second wave.
Peter Collins - how very PC of you. Sorry, couldn't help it.
I forgot about the kids - again, slightly hard to take when they pass but fair play to them. There was a mile race before a 5 miler I did a few months ago and the kid who won it came in pretty much dead on 5:00 and he must have been under 13 - most would find it hard to compete with that.
I had a run of only one female (not neccesarily the same one) finishing in front of me for several parkruns in a row and I'd never finished ahead of all the women in any race so it was a bit of a target for me. I eventually did it, but probably more by luck that a couple of the faster females weren't there that week. Apart from that - I have absolutely no chance in bigger races so it doesn't bother me.
As someone above said, I think it's harder to take when someone in their 60's / 70's overtakes you. However, you only have to look at the rankings on power of 10 / run britian to appriciate how fast some of them are and be in awe of them.