Ok, great response, I should clarify a bit here. I'm not trying to attack anybody's play type, and if you regularly play 20+ games you aren't somehow in the wrong compared to some other game type. I'm just trying to say how poorly anything on a 3rd edition skeleton handles the math getting pulled that far.
It totally also depends on your class. A Wizard is going to have a grand old time at level 20+, 30+, etc. a Fighter is going to be hard pressed to have something different to do at 30 vs. 20 vs. 10.
I do disagree about your estimation of levels, though. I mean, at 3rd level your pure casters are pretty crap, you either start a fight, surprise/win initiative and Color Spray/Grease or you lose initiative and something very nearly kills you in one attack.
By 8th Level said pure caster might have his 4th level spells, has multiples of 3rd, blasting Sorcerers have come online, plenty of good control spells, etc... but nothing changes. You either win initiative and contribute or you go late and the murder happens to you.
I have run something like 10 campaigns to level 15+ and three more to 20+, one of which was Wrath of the Righteous, with Mythic (which did not help anything at all whoo boy) and I have never really found anything great happens at higher levels (let's say 14+) the "rocket tag" analogy is extremely accurate, defenses can't keep up with offense in any meaningful way. Either your party uses stuff like scry and die, or it happens to them, or they roll over any sort of normal encounter that doesn't take into consideration Project Image, long range spells, or all the other stuff available at high level.
Like for once it would be nice if an adventure was like "this 20th level wizard badguy isn't screwing around. He'll use Wish to steal you party's spellbooks, take families and friends hostage, has Greater Planar Binding, Gate, and Leadership (with a Summoner ally) going, and you fight him in an airless, lightless void he has specifically prepared where he attacks at random from 1000+ feet away with the many, many magic items he has created for this purpose. If the party wins, his Clone activates, Contingency breaks a Staff of the Archmage at the party's location, and an underling receives a missive to head to a neutral deity's temple to pay for a True Resurrection for the Wizard when his Clone dies."
I guess my point is that if you want ridiculously high level adventures, there was a 0 Edition book that handled that, 2nd Edition D&D is fantastic for that (waves at Athas) and there are games like Exalted that exist to give that feel.
When people talk about 20+ Pathfinder, or 3.5, or anything on that chassis, I get this mental image of putting rocket fuel in a pickup truck.