Honestly I don't know if levels are a good idea these days.
We have that discussion about twice a year, if I remember right.
Levels are a terrific and efficient way of measuring your progress ("you're ready for Dreadlands now") and communicating your progress to other people ("we need a level 45 for LGuk"). It breaks down at level cap because you usually encourage progression even though you didn't allow new levels, and people try to substitute new "levels" to the capped level. Those we call "ilvl" these days. You don't hear "need level 104 tank", you hear "need tank for HM, ilvl 660 req".
My pet design does away with the concept of level as a basic newbie scale. You don't have a level, you have an apparent level which is computed from your gear, ability ranks, unlocked AA, whatever is in your game. So you're a "level 56 warrior" not because you've accumulated 56 bars of XP, but because you've accumulated 34 levels worth of gear, 15 ranks of abilities and 7 ranks of AA. Or maybe it's 29 in gear and more AA because you've grinded achievements and not done dungeons as much. It's open-ended, and allows an universal scale of measure (if you've done your design work carefully). It tells you what is easy, ok or hard for you, but does not dictate what might be useful for you. Of course, there are builds that are OP for their apparent level, but whose OP fades after a while (because the item power doesn't scale as fast, or because the other AA you skipped in favor of those that were OP aren't as OP compared to their stated contribution).
The biggest problem of that design is the catch-up mechanics when a new expansion arrives. If the lower level of the expansion is too high, people have a difficulty fording from previous to current expansion; if the lower level is too low, a lot of it is wasted on people. WoW has two progression scales (level, then ilvl) but resets completely both because you cap the first easily, and you get handed "free stuff" to cap the second when the expansion launches.
But actually, the biggest problem isn't the game, it's the players. The players "know" levels. Remove levels, and they go "uh?" on you.
Usually what happens after a few years is your newbie zones are empty, and new players are left with no one to play with.
That's a different problem, and one which has actually nothing to do with levels per se. You need
1) Stuff to do in "lower level zones"
2) Reasons to group with any character that has stuff to do in that zone
(meaning what you do in those zones is also useful to lower levels - it's just that you didn't do it or have reasons to redo it)
GW2, for instance, has stuff to do in lower level zones (WBosses). It's just that it has no reasons to group.