Vanilla -> TBC -> WolTK Progression
[/color]Progression private servers are not uncommon. A server starts with only the initial raids for that expansion, and opens up later raids as they're scriped. This allows players to "progress" from raid tier to raid tier much like how guilds did it in retail. Primal takes this concept a step further and plans to not only progressively open up later raids, but to progressively open up later expansions.
PW runs the WotLK 3.3.5a client, but is currently in the vanilla "phase". (Nearly) all content that wouldn't have been available in vanilla WoW is not available on PW yet. This includes zones, races, classes, professions, and abilities that were added in TBC and WotLK. When they switch to the TBC phase about a year from now, the TBC content will become available. Roughly two years after that, the server will enter the WotLK phase and that content will be unlocked. As the server has already been online in "vanilla" for about a year, this is roughly the same timetable that retail went through.
Within each phase, raid progression happens similarly to other progression servers. At the time of writing, AQ20 has just opened and AQ40 is expected within a couple months. Nax is expected several months after that. Each subsequent expansion phase will go through the same sort of progression, with higher raid tiers becoming available as they're ready.
How is PW's vanilla phase different than "true" vanilla?
[/color]Because PW is running the WotLK client, the basic mechanics have to follow that version of the game. It's certainly faster/easier to level than 1.12.1. You can hit 60 in 5-6 days /played without trying too hard on PW, which is about half what it takes on vanilla. Increased player damage/healing output from tweaked abilities and gear also makes leveling and dungeoneering easier.
The difference between true vanilla and PW gets greater the higher your level, and this caused a problem for raiding. A level 60 on 3.3.5 is so OP compared to 1.12.1 that raids required tweaking to actually provide a challenge. The solution comes in two parts: Raid bosses have their health increased dramatically, and players have their healing spells decreased by 66%. The boss buffs start out ridiculously high, and are slowly dialed down over time until the difficulty is acceptable. The healing nerf kicks in at level 55 and only applies in raid/instance/BG zones.
Class talent trees are locked out below the 7th tier, and only one point can be allocated in that tier. This is unfortunate for some specs which have the really juicy talents farther down the tree, but it only really screws the same specs that got screwed in vanilla (feral DPS, ret pally, etc).
There are other differences thanks to the 3.3.5 client as well - the biggest being the Random Dungeon Finder. It only provides a daily random dungeon goodie bag up to level 55, but is still a quick way to get a group for lvl60 dungeons. Guild banks are in place, as are post-vanilla quest hubs (like mudsprocket), FPs (like ratchet), and transportation routes (Auberdine<->Stormwind). PvP Arenas and the Achievement system are active. Dual talent trees are available after level 40, but the 1000G price tag can be prohibitively expensive in PW's vanilla-grade economy.
Cross-faction PvE!?!?
[/color]Private PvE servers are rarer than hen's teeth. PW takes this a step further and allows cross-faction interaction. This means the horde and alliance can speak to each other in chat, mail items to each other (all AHs are linked), join the same guilds, and even be in the same party/raids. This probably seems like sacrilege to many WoW players, but it goes a long way toward overcoming the problems associated with a lower population server. Management has promised to separate the factions when either the population gets high enough to sustain play without it, or when TBC is released (whichever happens first). At that time, faction transfers will be made available so guilds are not split up.
It's worth mentioning that just because the players on the other side are friendly, doesn't mean that NPCs are. If you walk into the opposing faction's cities or quest hubs, the guards will still attack you. And don't expect your friends on that side to help you either - assisting a player in the wrong city is considered "exploiting" and can earn you a temporary ban.
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I think that covers all of it. If you have any other questions or think of another unique feature of PW that bears mentioning, please let me know.