This thread has been very helpful in thinking about what the things are in the game that bug me and potential solutions for those things, so thank you all.
One thing I sort of came to while discussing it with a friend is that the big problem for me is the pace of PC capability and complexity increase and how that impacts verisimilitude, world building and adventure design as a coherent thing. I realize that the NO ADVANCEMENT thing is a tough sell, and was thinking of ways to find a middle ground and think I might have stumbled upon one.
So, the leveling mechanism is sort of a hybrid between milestones and "earned" leveling. The idea is that it takes a number of XP "tokens" equal to the level you are aiming for to get it. You spend these, so your total is always going down. You get one token for showing up, so every player in a session gets a token. If the session happens to be the end of a successful adventure, the players all get a bonus token. Also, once per level, a player can individually earn one token for reaching a personal milestone, resolving or advancing a subplot or other "role playing" things. If you are killed, in addition to whatever else it costs you must pay a token to get raised, and things like major alignment violations, breaking your paladin vows or whatever can also cost you a token. The idea is that PCs will level a little more often than once every number of sessions equal to their current level. If you started at third, for example, characters should reach 4th in 3 session (assuming a complete adventure in there). When they are 10th, it is going to take more like 7 or 8 sessions, depending on number of discrete adventures and subplots, etc.. What this does, and why i think it might help solve my problem, is it increases the time between levels as the PCs gain them, so there is time to explore those levels -- both from the players' perspective but also as a DM. Having 3 months or so to really get into what an 11th level part is and can do is more appealing to me than dealing with the quick leveling and wonky CR (and therefore XP) calculations of such characters.
Thoughts?