I'm an admin on
DartMUD, but I haven't seriously played a MUD in two years, just a bit of dabbling about to see what's out there.
Consistent with the last five years, the largest number by volume are DikuMUD-derivatives, particularly easy-to-set-up variants like Circle and some of the more advanced, flexible variants like Smaug and ROM. Merc has largely been superseded by the aforementioned two. LPMuds are still alive and kicking (DartMUD is an LP), particularly the Discworld MUD. By volume, hard to say unless you use a ranking site (e.g.
Top Mud Sites), where the "pay" muds from Iron Realms tend to dominate.
There's also some upstart codebases, like
Cold (with its own interpreted language; sort of a spiritual successor to LP). I've not tried them out, since they tend to mostly be in heavy development mode. There's also the Aber-derivatives (iDirt, Music) if you like puzzle-based gameplay. I used to write puzzles for a British iDirt-deriv (now under the Music codebase),
Cryosphere.
From what I've seen, the Diku crowd has tended towards going gamier; adding more and more levels (e.g. AvatarMUD, 50 mortal levels, 999 Hero levels, 999 Lord levels,I think) and more complex remorting systems. The best remort I've seen has you 'fuse' your body with another person, creating a slightly better hybrid for both parties involved to play in their next run up the ladder.
In the pay-for and LP crowd have, from my perspective, gone worldier. On Dartmud, we've been working on systematizing our mechanics so that things work together properly and adding new crafts and worldy bits (e.g. a constellation system). The Iron Realms games also seem fairly worldy, but I've not been inclined to set foot in them.
I haven't seen much of a direct push towards WoW's model of gameplay; on one hand, you've got your Dikus with their endless level grind and on the other you've got Aber/Dirts based entirely around puzzles/quests. However, I haven't seen, personally, a major mud that adopts WoW's constant-questing model.
PvP is mostly the same as it ever was in MUDs, with most Dikus either having free-for-all "war" periods and specified "arena" zones with draconian penalties for attacking other players outside of those specific circumstances. There are a small handful of pure PK muds, but I'm not seriously familiar with their gameplay or communities. On Dart, we have (and will always have) permadeath and unrestricted PVP (codewise, there's serious social constraints and some actively-enforced rules to prevent abuse), but it's not a popular feature in the MUD community at large.