The first problem that I ran into with the new version of Dungeons and Dragons is that the experience rewards are simply nonsensical. And for some reason, nobody seems to be aware of this.
(Actually, this wasn't the first problem that I ran into with the new D&D... Having to buy miniatures just to run combat with all the new minatures-centric feats and rules was the first problem. And I'm still irritated at this particular charge of things...)
For those of you who are unaware of how badly skewed the experience base is with 3rd Edition, below is a simple sort of comparison, based on 1st and 2nd Edition rewards for killing the most basic of monsters. Orcs.
In the earlier editions, orcs were worth somewhere around 20 exp. per kill. (Specifically, they were calculated as 10xp + 1/hp or 15xp, plus the various gold and so on.) If you figure that it's going to take the average 1st level character approximately 2,000 exp. to get to the 2nd level (I figure this is fair, seeing as we have to accomodate the fast ascent of the thieves and the slow progression of magic-users), it's going to take somewhere in the neighborhood of 100 orcs, per player, for a character to advance in levels. Okay, this is some work. It's going to take a while for the characters to advance in levels. If you figure that you've got a group of 4 characters, this translates out to a solid 400 orcs, give or take.
Let's compare this with the underlying theories of 3rd Edition, with the various Challenge Ratings and Encounter Levels. Orcs are 1/2 CR, which translates to being 150 exp. per thing. With 1,000xp per level, that's 7 orcs. So, for a party of four, that's a whopping 28 orcs. Not only are the orcs worth far more, the levels are far easier to attain, and the result is that advancement is bizarrely easy and fast.
(What's funny about this is the weird contention in the Dungeon Master's Guide that characters should level after 13.3 encounters. Taking this by 3rd Edition, that means that four characters should face off against 2 orcs at a time, 14 times, in order to level. Taking this by 1st Edition, this means that the same group of four characters needs face off against an average of 30 orcs at a time to keep with the same number of encounters...)
There are more logical variant rules available in the Dungeon Master's Guide (namely, setting the experience according to how difficult the encounter is, rather than following a solid scale), but these seem more applicable to higher level characters. (I have yet to calculate how it would function beyond the low levels, seeing as I'm trying to balance the low end before I try to monkey with the upper registers.) If you use this system, however, it still ends up being notably faster than 1st Edition. Sure, you end up handing out about half the experience, if done right, but that's still eight times faster than what we used to deal with.
At the moment, I'm still working on what kind of experience scale to work from, but it's becoming increasingly evident that the existing advancement scales are unworkable for anyone that wants to keep a game going for any length of time. (Yet another thing to grumble about in another post at a later point...)