I find that people who have not gone that high and are from sea level are particularly susceptible to hypoxia. Doesn't have to do with being young or old, or even fit or not, even though those play factors, the conditioning is usually the biggest one. For people that cruise 10-12K without O2 day to day it's usually a non-issue, but don't discount this if it's not your normal operating procedure. Potentially big deal.

Otherwise, as people have said, a circle usually kills what little climb performance you have up high. With a lot of updrafts, you can "find" the rising air on a hot day, which is also fun.

It's nice to get up to 10K or higher on a hot day and experience the cool air.