Umm, I really, really, really, don't want to see D&D 4.5, ever.
I prefer them continuing to update rules in the MMX/ PHBX / DMGX books.
And I hope it will be a long time indeed till they release D&D 5E.
Except that if they decide to ever do wholesale changes (like to multiclassing) it will probably invalidate the previous system.
The reason to put out a new system is if it is superior mechanics-wise.
I'll give some examples:
1) Replace "until start/end of PC's/foe's next turn" with "until end of PC's next turn" for ally buffs and with "until end of foe's next turn" for enemy debuffs (or anything negative on the enemy which is not save based). That speeds up the game because every one of your positive effects ends at the end of your turn and every negative effect on enemies lasts until the end of theirs. As is, there is quite a bit of game slow up as people figure it out, or forget it, or whatever. Better yet is to make all negative effects end on a save. That's even better.
2) Temporary hit points clash with minion rules. One set of rules or the other (or both) should be adjusted to take into account the other set of rules.
3) Improve multi-classing. Make it a type of dual classing. In for a penny, in for a pound as opposed to allowing players to cherry pick the best options (like healing one's self).
Granted, each person has their own list and will disagree what is good and what is not. But, WotC could use polls and other mechanisms, and have several years worth of data and research as to what works and what doesn't.
Alternatively, take the rules changes and the errata, clean up the core books with them, and sell them as .PDFs. It's faster to search a .PDF than it is to look something up in the book anyway.
But, it's annoying to look something up in the PHB and overrule a player or some such, and the next day find out that the rule has since been changed. I'd prefer one source of rules, not rules spread over 10 books (I'm talking about core rules here, not rules specific to a new class in a new book).