A grind in different clothes
But, to turn it around and look at it differently, it has the same old problem as before. You're free to do whatever you want, as long as whatever you want to do is kill mobs and get coins, or answer lore questions, or find chests, or collect bits of tigers for a panda chef. If none of those things are your preferred type of content, then the Timeless Isle isn't going to be very exciting for you.
A good friend of mine, Alex, tweeted back at me in response to my happiness saying that it was just a grind in different clothes. When pressed, she followed up, saying that maybe it was because she's a healer. I know Alex well, and when she says she's a healer, she's really not joking. She has every healer at max level, and all but her Druid are raid geared to a good normal-mode level. Funny enough, as someone who loves healing so much, she hasn't had much time to spend gearing up her off-specs, or her hunter, so the Timeless Isle isn't really much fun for her. Hitting like a wet noodle against the mobs is not much fun.
There has been a general lack of content for healers and tanks in Mists of Pandaria, outside of raiding and PvP, and I'm not sure that Proving Grounds have done much to remedy this. People who play those roles miss dungeons, and having meaningful content to do that caters to their chosen specialty. Proving grounds don't replace dungeons. The Timeless Isle doesn't replace dungeons. Scenarios, heroic or otherwise, don't replace dungeons.
I suspect that that is what Alex misses, simply because it's what I miss. I don't want to have to spec my tanks and healers into DPS to enjoy the new content, I want tailored content for me in those roles. And yes, proving grounds are cool, but they're not a replacement for dungeons. Dungeons weren't broken. Stop trying to fix them.
The bright side of the road
But nonetheless, as someone who plays a lot of shaman and paladins and DPS, I'm having a (pyro) blast on the Timeless Isle. The principal reason for it, I think, is the lack of structure. It's really fun to me to decide that today I will be the nemesis of Gulp Frogs, and wipe them off the face of the planet. And it doesn't matter whether that's the objective I'm meant to achieve for whatever quest, as even if I do the prescribed tasks, I will likely get just as many Timeless Coins from my Gulp Frog murdering ways.
Or I can be more efficient, and restrict my fiery ire to the emissaries of Ordos in order to make a ghostly panda emperor like me. But the great thing is, unless I'm really desperate for one of the things that Shaohao rep allows me to buy, it doesn't matter if I don't.
Maybe I have some kind of Mists of Pandaria version of Stockholm Syndrome, but I'm really happy that the Timeless Isle doesn't lock useful things behind rep grinds. Instead, it practically rains 496 gear, with the difficulty coming when you want to get Burdens of Eternity to upgrade that gear to 535. But that's not a big deal, hunting them out is still fun to me. I feel like Indiana Jones, leaping from an Albatross to parachute in on some minotaurs and steal their treasure.
And even Alex has found something she likes to do. The Celestial Tournament, for pet collectors everywhere, has piqued her interest, and while she's a little frustrated at the luck-based element of it, she's having fun.
The army of alts
The biggest thing I like about the Timeless Isle, though, the number one top of the range thing, is how easy it makes it to gear alts. My main PvE shammy had decent enough gear, although Raid Finder Fatigue had well and truly set in during Throne of Thunder, so thanks to that and some bad luck with normal mode drops, she was seriously behind the gear curve. Cue Timeless Isle and a couple of Burden pieces, and she's away. My paladin, too, was way behind on gear, simply because up to now it was really pretty hard to gear up a tank. But a few hours on the isle, and she's at ilvl 496, ready to step into Siege.
Let's not talk about the weapons and the shields, though. If I have one major criticism of the Timeless Isle, it's that for some reason, Blizzard's designers have a "thing" about weapons. The only ones you can get off the isle are ilvl 476, not 496 like everything else, and cost a whopping 10,000 timeless coins. They're not well itemized, they're not plentiful, so you have to go for "close enough" rather than "this is good for my character". Blizzard, what is up with that? Why can't these weapons at least be 496? Why do you guys feel that we have to earn decent weapons purely via luck from boss drops? Seriously, it drives me mad. My poor paladin has an ilvl 463 shield. She can't go and tank in Siege with that equipped, even though she has the ilvl. And, although I say so myself, even though I'm a good tank.
But maybe you dislike how the Timeless Isle makes it too easy to gear alts. I can't agree with you on that, I'm afraid, but I can sympathize on healer frustrations, on not wanting to grind out mobs, on finding the whole isle something of a waste of your time. And if you really wish there had been more five-mans instead, I agree. I'd swap every fun moment, every chest, every albatross, every toy, every celestial, for some more five-mans.