Yes, MoP is mainly over, but there is speculation of there being a 5.5, mostly to bridge the gap between this expansion and the next, and to clean up any stories left.
And Garrosh is the last/main boss of MoP, though Blizzard can go back on their word and make filler raid. Who knows.
MoP is without a doubt the best expansion since TBC.
I think MoP has the best gameplay of all expansions, and the most amazing graphical fidelity of course. I think the only reason I haven't really enjoyed MoP and quit since 5.1 is probably because I couldn't get into the lore and the setting of it. It felt odd and not serious enough for me.
Also difficult content for few players is lacking but this has been a problem since the end of Wrath. I want some hard content to do with up to 8 people or less and more of it! (And it has to be different, doing those hard dungeons for transmog gear is the same old dungeon but just harder.)
Also questing is too simple and easy as well, I like the journey and think less about the destination. (Problem since Cata.)
If you're looking for something to do throughout this xpac, then you won't be let down at all, I myself no longer play but this wasn't because it was bad at all, give it a go for sure.
HOOKED ON DIABLOL, GOOD TIMES ARE BEING HAD
If the game started to feel stale to you, most likely you'd just been playing Warcraft too long and you'd had enough. If that's the case, you won't find it any better now. It's still very much the same game as it's always been. As far as comparing it to other expansions, I'd say it's better than Cataclysm, but still inferior to the rest.