My friend and I who played WoW avidly for years and quit right after the Deathwing patch are thinking about getting back into the game and I have a few questions.
1. Is this expansion fun/worth it to get back into?
2. What are the most fun/strongest classes for healing currently and why? (My favorite thing to do in game is heal raids)
3. How are the new raids dungeons compared to previous expansions raids so far?
First of all, it's useless to ask these questions and base your decision on what people like me say about it.
1. That being said, you have to ask yourself the question about what you want to do in WoW. I used to be pretty diehard in vanilla/TBC and since then more casual with every expansion. I enjoy that a lot, and MoP has plenty of stuff to do. If you want to go 5x a week raiding, you should ask someone else.
2. It does not matter at all which class is the best. I've played priest 80% of my wow time. In MoP I decided to roll mistweaver monk and see what's it about. In heroics I like it a lot, in LFR I don't have a clue what's going on and I've not done normal/heroic raids yet.
3. Can't answer that.
Last but not least, if you like the game in general, just buy it and play it. Maybe you will get bored with it within a month, still you will have plenty of hours played for the money you've spend.