It's a pretty interesting question especially considering that recently there has been a lot of interesting in pre-TBC servers. I've written a quick look at the pro's and con's of each WoW version from both a server owner and server player standpoint.
Vanilla WoW: 1.12.1
Pros
- Player nostalgia - people miss the good ol' WoW vanilla days of world PvP, raiding classic instances; basically that awesome time when most of the stuff was brand new and we first experienced it.
- Balance - people prefer old talents/skills than new ones that have been added.
- Static version - don't have to deal with updating server cores to a brand new patch. Also makes it easy to develop on because you just deal with that one version and there is a realistic possibility to have a 100% working emulator.
Cons
- Distribution of files is a major issue - It's extremely hard for players to download the original WoW client. Finding patches isn't hard but the original WoW client is so scarce on the net now it makes it really tedious for the player to actually get on the server. (Challengable?)
- Outdated cores - Currently, there is the TrinityCore Zero project that is working on having a working 1.12.1 core but it is very buggy and is still under heavy development. Other options include classic versions of MaNGOS but these are nowhere near to the stable featureset we have in modern emulators.
- Content saturation - Vanilla WoW doesn't offer as much content to work with; newer expansions expand on WoW in numerous ways while also adding in the original WoW content so they are always above Vanilla WoW servers in that they have more flexibility to add new content.
The Burning Crusade: 2.4.3
Pros
- Distribution is easier than 1.12.1 - much easier to find the 2.0.0 trial clients and The Burning Crusade downloads.
- More content to work with - additional instances, brand new races and models allow for server developers to have more flexibility in changing & add new things.
- Still some nostalgia - TBC is kind of in the middle in terms of nostalgia, it's not exactly Vanilla, but it's definitely not completely Wrath of the Litch King) (Kind of a weak Pro, but valid)
- Cores are more updated and provide better working features than 1.12.1
- Have similar advantages to 1.12.1 in terms of static version because you still have the realistic 100% completion goal just as with Vanilla WoW.
Cons
- Not exactly Vanilla WoW
- What's the point of going TBC when you can just go straight to WOTLK and get all the advantages of having all of that content?
Wrath of the Lich King: 3.1.3 (*current)
Pros
- Latest version - you are capable of creating a server that can be mimicked to the exact way WoW is now!
- Variety of core support - from Ascent based emulators to MaNGOS based emulators, several cores offer their support for this latest revision
Disadvantages
- Not in perfect working order, and highly unlikely it will ever be fully functional as it is constantly being updated. Players will always have to deal with a semi-functional server.
- Lots of competition (recent version, obviously why)
- Not as unique as Vanilla WoW
- Some think that Wrath just sucked?
Please post your feedback, your own pro's & con's and feel free to challenge my own.
Eagerly awaiting for your responses.