I've read many aviation articles as we all have, and done the human factors exams, both RA and GA. Here is the first mention that i can recall about hypoxia below 10,000, with the exception of night flying, and heavy smokers. These must be extreme cases for people to be suffering from hypoxia at below 10,000. Magazine articles would suggest that in the US they don't use oxygen much below 12,000 and the human factors syllabus mentions lenghts of useful consciousness at 15000. I must concede that I don't know the legal height for oxygen in the US, but they don't seem overly fussed below 12000.
A 10000' ceiling is absolutely fantastic for getting over mountains, a few clouds and just getting about comfortably on hot days, and I find the radio easier than below 5000 as you don't have to change frequency as regularly as when travelling through CTAFs. IFR traffic is on their level and VFR on ours, so if everybody is doing the right thing, there is adequate separation. The higher altitudes are great for going places in a hurry (if the wind's right).
Perhaps one area where more training will be required if the limit is lifted to 10000 is the IAS vs TAS and pressure heights. At lower altitudes IAS and TAS are very similar, at 10,000 there is a significant difference, and is a concern if an aircraft is being flown close to VNE. The RPTs can be a concern, however they are just as scared of us as we are of them. They hate going below 5000 because traffic can be coming at them from all directions. The RPTs seem to make their calls early, and a little bit of communication goes a long way.
The other big factor is the cold. It can be bloody cold at 10000 and you need a good cabin heater, oh and the scenery changes very slowly at that height.
Cheers Ferris