Allowing for variable raid group sizes
No matter what MMORPG a player goes to, a persistent problem with end game is there are never exactly the right number of raiders to give all competent raiders a group. Hard limits on player number will always leave stragglers, which is something completely outside the control of the player base to fix.
While it may be difficult to perfectly balance and will undoubtedly require a change in the loot system, I feel that allowing parties to take excess players along with them would make the community a much happier place in the long run. Raiding is about doing something with friends, and nothing is worse than seeing your friend leave the game because he got left out of the fun. There really is no good alternative progression for a raider other than raiding (at least no one has found a good substitute yet).