(Hm, apparently this has been sitting in my drafts folder for over a month but I never posted it... might as well.)
One of the big changes to the game in Mists of Pandaria is that a lot of things that used to be bound to a specific character are now bound to the account/player instead. Now, as a general rule I have to admit that I'm not a big fan of this. I always feel for Bhagpuss when he's trying to explain to people why it matters that characters are their own - well, characters, and not just different puppets for the player to mess around with, even if you're not a roleplayer. I've always felt the same way really and never thought of that attitude as very peculiar... but apparently there are a lot of MMO players out there to whom these things don't matter. Still, to me it feels kind of wrong that a pet that I buy on one character should suddenly be available to any of my characters - without any kind of explanation anyway. However, I think WoW gave up on in-game explanations for convenience features a long time ago, probably around the time they introduced the dungeon finder. Anyway, with that out of the way...
Now this is the one thing I don't mind becoming account-bound, because achievements have always been aimed at the player anyway. They don't really exist from an in-game point of view and are thus irrelevant to individual characters. When The Old Republic added an achievement system last year, they made it account-wide right away, and it's been working very well in my opionion. Once you're on your tenth alt in that game though, you won't see many achievements pop up anymore as you level, simply because you've already done it all - though there are still codex entries to collect on a per character basis (which are similar to achievements, but separate).
From that point of view, I like WoW's current idea of having both account-wide achievements (for the more difficult ones) and character-bound ones (for the easier ones). It combines the benefits of not feeling annoyed when you get a tough achievement while on an alt ("Why couldn't I have got that on my main?!") while maintaining a steady stream of flashy dings throughout the levelling process, even as you explore Mulgore for the fifteenth time.
The only problem I have with it is that the implementation in the achievement panel is wonky. For an example of what I mean, take the World Explorer achievement. That's labelled as account-wide, fair enough. The associated sub-achievements however are not, yet they still light up as already done even on a new character. Only if you dig deeper and check the sub-requirements for those achievements, they will show up as incomplete, and if you complete them, you'll get an achievement pop-up as if you'd never done it before. This is confusing and makes it hard to keep track of character-based achievements that are related to an account-bound meta. I kind of feel that they should have been able to find a better way to do that.
Probably my least favourite new account-wide thing. It offers some convenience, sure - when my Worgen druid hit twenty, I could just pull out a random mount and use that instead of going to buy her a new one. (Well, strictly speaking I could have stuck with Running Wild on a Worgen, but I don't like that ability very much.) Not to mention that all my alts capable of riding have access to my Traveler's Tundra Mammoth now, meaning that they can pull out a vendor whenever and wherever they feel like it.
On the flip side though, that special connection between a character and his or her unique mounts is gone. When I looked at my new account-wide mount panel after having logged through a couple of alts for the first time, I was most surprised to find a Headless Horseman's Mount in there. I don't even remember when I got it, I'm guessing that one of my alts must have been lucky when I already didn't care that much anymore... but now I don't even know who it "belonged" to. And sure, I could ride my Amani War Bear on any of my characters now - but it'd feel wrong as they aren't the ones I earned it on. I just don't quite see the point other than bragging rights. Finding and earning the right mount for a new character used to be a rite of passage... but when you already have access to everything you've ever owned in the game, that journey is one that you can't really re-take.
Also, it feels like parts of the game haven't really been streamlined to take this new feature into account. For example if you do the Vashj'ir intro now, you still get a seahorse mount as a quest reward that you can't use and the only thing you can do with it is throw it away. (Delete a mount! The blasphemy!) I would've thought that the game should be able to recognise if you already have a certain mount and shouldn't clutter up your bags with pointless duplicates.
Like with mounts, I feel that there used to be a connection between characters and their pets, though I personally didn't perceive this as strongly as the link with mounts, as I've never been much of a pet collector. Still, this link is obviously gone now. However, I'm willing to be more forgiving with this one as I can see it being almost a necessity to make the new pet battle feature work - else you'd constantly find yourself running into rare pets on the wrong character. The fact that you can box some pets up and re-sell them also makes it an awesome way of transferring money between servers. Again, the only thing that bugs me is that some details feel unnecessarily clunky - such as that some pets are BoP until you learn them, at which point you can cage them and they become BoE. It's quite annoying when you already have three or more of a particular pet and instead of being able to sell it right away, you have to cage one of your existing pets and then learn the new one. Just seems... unnecessary.
This one I have slightly mixed feelings about. I can see the point of people enjoying the ability to carry the bragging rights onto all their alts, though personally I (once again) wouldn't much fancy using a title on a character that didn't actually earn it. Sure, I was majorly miffed when I only got "The Undying" on my alt and not my main back in the day, but pretending that it didn't happen and wearing the title on my main doesn't "un-do" what happened. I also noticed that some titles, while account-wide, have retained a level restriction, which just strikes me as really random. If I'm going to run around wearing "Hand of A'dal" on a character that didn't earn it anyway, why does it still matter what level that character is? Either way, I don't feel like there is much of a loss to me from being able to wear a different title - they never felt quite as special to me as mounts for example.
One reason I do like the new account-wide titles is that it takes WoW quite a while to give you any titles as you level up, and this way you can pick something from your existing collection while levelling. I don't mind using something silly like "the Love Fool" or "Jenkins" on a lowbie for example, as I don't have a particular attachment to these titles and it does give my character a little more definition.