News, News and News!
You'll use your experience of previous games, what you've been told, what you've managed to glean for yourself from literature and from mates.
The biggest single issue you're likely to be concerned about is immersion: 'this game's supposed to be really addictive' is a phrase I've heard more times now from random people I've spoken to than I care to remember, and my response to this is ALWAYS the same: that's nothing to do with the game and everything to do with the people playing it. Alcohol, cigarettes, sex, sport... all these things are addictive too, it's about your reaction when it happens.
If this game really were that addictive there'd not be a drop off in subs, so let's dismiss that issue and move on. What you do need to grasp, and this is absolutely vital, is that if you want to enjoy the experience as a first time player or a returnee, you should NOT do it alone.
Who you choose to play with when you begin is perhaps MORE vital than where you start playing.
Once upon a time, a Server selection really mattered, because everything you did and everyone you would ever interact with lived on the same server blade as you did: you were an isolated community.
As social networking has become an increasingly significant part of gameplay over the intervening years pretty much none of that matters any more: you can start anywhere and the whole game is available to you, if you know where to look.
Blizzard understand that actually it does matter a very great deal who you play with, and that this motivates a great many more decisions than perhaps it was ever aware of to begin with.
It has the potential to affect pretty much every single choice you make within the game. That's not just where you play, but what too: twinking alts, running raids, attempting challenge modes, making gold.
All of these are governed by not only by the people doing them, but the information they generate and that a new player can access when they begin their journey, or when they return after an absence.
Yes, I'm back to you, returning player, because your choice in starting again or coming back depends more on this than anything the game can give when it loads up in front of you.
It isn't Warcraft's job to hook you, it's your choice to be hooked, and that's got more to do with the people playing than it ever had at any point in the game's lifespan.
If you don't mind, my newcomer friend, I'm going to briefly leave your journey to one side. I want to talk to our old hand for a moment, because their situation is actually quite relevant at this point.
Past and present exist simultaneously. Cosmic, man!
Enemies come and go, but the basic motivations remain.
You know your reasons for leaving, and every individual's motivation is different. There is absolutely NO POINT in telling you that the game is better or worse than it was when you departed, because it doesn't matter.