I was gonna answer your post long ago before people did, with honest answers about the game.
I played WoW since day one, been researching about it ever since i heard of it (Warcraft 3 CD ftw!), i have taken big breaks from it, TBC for over a year, WoTLK for 10 months and Cataclysm for 10 months (Rift ftw for the cataclysm one >_>), during my play time i was always playing properly and hardcorely.. Cthun on farm on Vanilla, most bosses in Naxx vanilla, very fast early progress in SSC/TK, Ulduar Hardmodes very early on etc..
MoP is gonna be the same as TBC/Wrath/Cata was, something completely new, a full reset, a reason to check out new stuff, its like a huge content patch for WoW and its the reason many people return the first 2-3 months, its how WoW is and that will never change, WoW is indeed a great game, but for people that played it for a long time, its a bit boring, but when an expansion hits, its the same, new stuff to check and learn the first month!
Now you asked about Rift and its endgame.
This sentence expalins it well "Its the offspring of TBC and Vanilla WoW".
What that means:
1)Raiding mechanics are unforgiving there is no :
"Here is normal, you can stand in the fire for 2-3 seconds, just move out after"
Or
"Here is the hard mode, same fight as before, but move out of the fire the first 3 seconds or you die"
There is lots of:
"Yeah, you see the effect that spawned? I dont know what you are gonna do, get the hell away instantly or you die"
Not all mechanics are like that but you get the point, the damage/health ratio on many abilities of the bosses are simply unforgiving and failing on them stresses your healers much more than they should, assuming you can survive them.
There are also some resistance fights where resistance highly helps or is required, as example, think Ragnaros in Vanilla WoW and Princess Huhuran, not all fights are like that, but a few are, including the hardest boss in the game right now, Akylios (He wont be the hardest after March i think >_>.)
2)If you are used to WoW gamestyle, Rift will surprise you in many ways.
a)The most important part of Rift and the reason i cant jump ship.
The class system, its simply awesome to myself as a Rogue to have the power to switch specs.
In short example in WoW terms.
In 1.7, Rogue specs will be even more refined that you can have multiple options without hindering your raid, what does that mean, my main spec right now is Marksman, which is as it sounds, Marksmapship hunter in WoW, silence, big shots etc...
What makes Rift is awesome is, that my Combat Rogue (Bladedancer spec) is on the same character, and what makes it even more awesome..my tanking spec (Riftstalker) is on the same character, same as my bleed spec (Sin spec), and my Beastmaster (WoW-Wise, not Rift wise, talking about Ranger) same as my Support spec (Bard).
b)If you are used to the easy-mode WoW grinding that WoW became after Wrath of the Lich King, Rift will tire you.
The grinding isnt heavy, its just not easy mode,but they are making it easier and more refined patch after patch, doesnt mean its easy, you wont be wearing Hammerknell (The top gear) in 2 weeks, unless you get carried by a guild.
c)Rift literally starts at endgame, and this is really important to understand, there are many new players saying they dont meet many people while leveling the main reasons are that
1)A class covers most stuff.
2)Lots of things to do at 50, to roll an alt, doesnt mean you cant roll an alt, it just means if you want to finish everything on your main, it takes awhile.
d)PvP wise, many people are complaining about PvP, but thats because they look at it from the "5v1" prospective that they meet in Warfronts (Battlegrounds), the game isnt as bad as people make it sound. "He killed me in 2 seconds"
Yes people die in 3 seconds 5v1 if they dont do anything, same as in WoW, same as in every game you are 5v1 ..
I am wearing full PvE gear and i survive up to 5-10 seconds if i play back, and because of the gear level against some players i actually kill them (And die from dots...Fuck PvE gear!)
Generally dont read the PvP forums much, they exaggerate a lot.
Yes there is no competitive PvP in Rift (No Arena), but they have some weird plans for 1.8 apparently, just a rumor though, so cant say much on that.
Anyways, PvP is getting changed to make the progression, smoother for new players, the gear changes and the rank changes coming in a couple of weeks, ensure more "fair play"
The PvP grinding is heading towards WoW way, so it will be familiar to you but with a difference.
They made it so the new players will progress through gear equally while playing in PvP.
The lowest rank gear was removed, you start from Rank 3-4 Gear (I dont know when you played WoW last, but think of it as starting with 346 item level gear), then use the same items+ some Favor (Honor) to buy the rank 5-6 gear.. (Think of it..as item level 365 or so), then Rank 7-8 Gear (Think of it as item level 390 or so), but you need the rank for the weapon..But there are some crafts, and drops and changes in the game that will help you with that.
It just means what it means in WoW, the guy with the big weapon will hit you harder than you hit him, but you will survive enough to actually play.