High-level play is not something that I have a lot of experience with. In 3rd Edition my group we managed to--by the books--get to 14th level on three occasions. In my 4th Edition A Sundered World campaign, the party managed to get to level 15 I think, but that was only because I was leveling them up each session; normally we would get somewhere in the 6-10 range before we would have to call it quits.
In any case I am told that high-level 3rd Edition play is fraught with issues; spells and Hit Dice progression rates could wreak havoc on a DM's attempts to challenge the players. I think I was pretty lucky in that in my campaign the worst I had to deal with was a characters' greater teleport spell-like ability, though I was personally the subject of a pre-Revised Edition polymorph spell; going from human to minotaur was fun while it lasted.
From my very limited high-level 4th Edition experience I can say that my biggest issue was the slew of class, race, feat, and item powers. It made most characters fairly difficult to juggle pretty quickly, though thankfully I never noticed any singular game-breaking powers (though some combinations were pretty annoying). Otherwise creating monsters and setting DCs, even on the fly, was a breeze.
A slower progression rate will help alleviate the scaling issues from 3rd Edition, where by 5th-level a fighter and wizard could reasonably have attack bonuses of +9 and +2 respectively. I think that instead of doling out an almost uniform +2 to weapon attacks for everyone at the start (a cleric and wizard both have a +2 with weapons, really?), that they should just use 4th Edition's weapon accuracy system.
It could also apply to monsters, which would help shore up some accuracy issues and inconsistencies. For example, why do hobgoblins get a +2 to hit despite a Strength of 11, when drow get a +0? Human warriors also get a +2 to hit, but have a Strength of 12. It does not seem to be a weapon thing, as koprus have a +4 to hit despite only having a Strength of 15.
Getting a spell or class feature "most of the time" can lead to lackluster level advancement. 3rd Edition had plenty of classes with "dead levels", and Next's fighter and rogue already have their share. Personally I would like to see something else going on at those levels besides just hit points, skills, and feats (especially since the latter two are technically optional systems). Though, given 4th Edition's progression model this might end up being too much. This is one of those things where actual play experience will come in handy in casting a deciding vote.
I am interested in the legacy system. Though AD&D is specifically mentioned, it reminds me of 4th Edition's epic destinies, which provided ways to resolve your character's story once you hit the level cap; you might ascend into a god-like being, found a magic college, carve out a new nation, transform into a lich, and so on and so forth.
Legacies seem to be less, well, epic. You might take over a thieves' guild, build a temple, or establish a stronghold. Lichdom is still on the table, and while you may not be able to become an archduke of Hell you can at least be able to thwart one. Of additional interest is option of transitioning back into low level, by playing heirs, hirelings, or followers of your old character.
This was something I liked about the idea of epic destinies, having never actually been able to pick one or seen them in play. You could found a college, and then play a member of that college. Or, if a character became a lich it could be a villain for the next one. The problem was of course the 21 level entry fee. Chopping this down to 11 would make it much more likely that characters can qualify.