My personal experience on playing WoW after being gone for 4 years:
12/27/13 - 1/27/14
Initial Play Period:
I received a SoR a month ago and decided to come back to the game and play as a Retribution Paladin. I got a Get-Out-of-Leveling Free card with the SoR (Scroll of Resurrection), so I had a new class I'd never played before at level 80 (first lesson - don't use SoR on a class you haven't played before, it sucks).
I was part of a raiding guild when I left, so I hit their forums, got a guild invite and went to the gear grind.
Leveling from 85 - 90 in MoP was...fun. I'll admit that I've always hated leveling in WoW. I genuinely despised their quest content in every content expansion up until this point. It was a nice change.
As soon as dinged 90, I hit up Timeless Isle for free purples and felt like WoW had really taken some strides in helping noobs get back into the game more easily. I felt like I'd be back to raiding in no time! That's when I hit the LFR dungeons...
Three weeks later, I'm still at the grind about 5 upgrades away from hitting the next tier of raiding, which is still two tiers below where my guild is. I don't really enjoy the Ret Paladin, because it's heavily gear dependent and partly because every fight feels the same. The rotation is pretty simple, so after a while my eyes glaze over and I wake up 15 minutes later to a dead boss and find myself wondering how it died.
Instead of waiting 20 - 40 minutes to find an LFR raid, I've started using "Oqueue", which has helped a lot in finding raids more quickly. However, the gear grind is still very evident and even if I do catch up to my guild in progression (fat chance), I will have raided that instance hundreds of times... JOY!
Secondary Play Period:
Then, I had the bright idea of RAF'ing (Refer-a-Friend'ing) my roommate. "Hey," I thought. "This will probably be more fun playing with a friend." So, we rolled a Resto Shaman and Protection Warrior duo. I've never played a healer in any MMO ever, so I thought something new might bring some shiney with it.
The 3 times multiplier on XP + instant dungeon queues has really made this secondary play period delightful. The reasoning is two-fold:
1. XP travels at what I consider the perfect speed (you never have to do the same dungeon twice)
2. Class familiarity progresses quickly enough, to where everything is still new and somewhat challenging (especially when you go from Cataclysm dungeons to MoP dungeons)
Healing has also been fun because it's not annoying. I went from a class that had me spamming and running around every second of every battle, to a class that will allows me some relaxation. That is until the tank pulls everything in the room and I'm utterly panicked for 3 minutes.
I'm still not looking forward to the Raid grind, but I think it'll be different with the Warrior/Shaman for two reasons:
1. Instant queues
2. I now know a better way to pursue progression than spamming LFR (thanks Oqueue!)
TLDR; Leveling in MoP isn't half bad. Retribution Paladins are spammy. Healing is easy. RaF is fun. SoR is good for classes you've played before.