My experience so far. (3-4 months in)
Since I play alliance, Horde experience will vary.
In the beginning, the server will feel abandoned, because majority of players are 90 and doing 90 things. That's completely fine, because the journey to 90 is not that difficult if you played this expansion before. You can que RDF during your leveling, but don't expect to join one unless you're playing with 5 friends. Overall, people will be helpful, and if you ask appropriately, they will aid you. Just don't beg, and you will be fine.
You will encounter some disconnections during your leveling, and sometimes fall through a floor or two, but the website has an unstuck feature, which nullifies that issue. (falling through the floor is still an issue) Some locations have really buggy areas which will play with your mind, and some of the flight paths will force you to relog to regain control of your character. Fortunately, all of these things have a simple cure until they are permanently fixed. Someone thought ahead when they released this.
Not all quests are fucntional, but the majority is doable, and some even finish the quest automatically if you wait a moment. There is some convenience in that, and I hope they apply this type of function to all non-working quests until they get fixed. Most dailies work as well. Infact, there is a popular way to finish 78-80 by doing the argent tournament, seeing as they are all available to complete.
I believe half of the Cata dungeons do not work, as i've tried entering them and no dice, but the ones that do work, work well. I have not experienced a single dungeon in Pandaria yet, but that was by choice, but there is plenty of shouts for RDF at level 90, so I assume most are functional.
Wintergrasp is not functional, neither is tol barad (correct me if I'm wrong please), which is a shame, because some of those honor quests would help new characters catch up.
surprisingly, we have a market, and I feel very confused by this, but there are demands for several things out there. I've recently provided some food items to someone who needed them, so there is value in gathering if you choose to do it. Transmog is very popular at the moment, and people are always trying to sell their things for someone else's looks. At the moment, there seems to be a high demand for leatherworking and blacksmithing, so for anyone looking to join the market, this would be a good start.
The people are generally friendly. Everyone always tries to start up events for world bosses or heroics. Everyone seems to be welcome, as long as you can pull your own weight, unlike icecrown. We have guilds who invite new players to help them catch up, and recently, I've spotted a guild which is trying to revive this community with old players joining. So, overall, the activity is very visible, and I am seeing more people daily.
After hitting 90, I am bombarded with things to do, and unable to decide my destinations on a daily basis. I generally like to stretch my time, but only if I found something profitable to do. Recently, I've spent far too much time on this server, because I've found several things I can do, but not enough time to do them all.
Bgs are very active. Someone is always spamming to join and I know a handful personally that do join daily. Being cross faction, it helps for a que to begin. World pvp is a joke, but you already knew that going in. (yes, people do attack you if they feel their time is not wasted)
Overall, from my experience, I've found a reason to stay. It's a different environment than icecrown, and everyone secretly already knows each other, which makes for some pretty humorous conversations. People will simply appreciate that you have chosen to play here and fill a role someone needed to. Worth giving it a try in my opinion.
As a side note: "is this a dead server?" has become a meme here. It basically starts a conversation and gets global chat to become active. Try it sometime.
