If I had to summarise my overall impression of the new story content on Makeb in three words, they would be "great but short". Even knowing full well that we were only going to get a single new planet in this expansion, I still felt a certain sense of disappointment when I completed the last step of the new story quests and realised that they only lasted me for two days and that it's all back to raids and dailies again now. I suppose in a way that's just another positive judgement of the story, that it immediately leaves you wanting more. Then again, I think those of us who have stuck with the game this far have probably accepted that it can't be all story all the time, and have genuinely embraced the other aspects of the game too. Still, it's always nice to get more.
I thought it was quite obvious on Makeb that Bioware has taken player feedback about various aspects of the existing levelling game very seriously. Take travel for example: while all those mesas make the planet feel huge and you can spend plenty of time cruising around, the main story will always move you along very swiftly now. Just fought your way through a big enemy base and finished off the boss there? Oh look, it's a convenient shuttle that will immediately take you back to where you can hand in and follow things up with the next part of your story. I'm always wary of overdoing it with the shortcuts in an MMO, as I like travelling and getting a feeling that the world is vast, but I do think that in context of the planetary quest line it makes sense to allow people to move on quickly, so as not to disrupt their flow too much. In a similar vein there seem to be fewer (voiced) side quests now, so it's easier to remain focused on your main task instead of getting distracted by some random side mission.
Another thing that I found interesting and positively surprising (and this is something that multiple guildies commented on as well) is the dialogue. It's not that it was bad before, but especially on Republic side, many of the light side dialogue choices came across as kind of... bland (unless you were a smuggler). This has noticeably been improved on Makeb, and now even Jedi and troopers get to make amusing quips on occasion and just generally display more of an attitude.
The only thing that was a slight let-down for me about Makeb's planetary story was - and don't continue reading this paragraph if you want to avoid spoilers - how I just couldn't reconcile the story arc with my out-of-game knowledge. Plenty of story lines in the game are about planets being in terrible danger, but somehow that never really bugged me even though I knew full well that I was going to save them eventually. With Makeb however they took it so far that I just couldn't suspend my disbelief. Everything was about how the planet was supposedly about to blow up, and here I was sitting the entire time thinking: "But it won't. It's the new endgame planet. This doesn't make sense." I kept waiting for some kind of quest to stabilise and literally save the whole planet, but it never came. My character "fled" from Makeb, fully expecting it to tear itself apart, and then it was just: oops, I guess it didn't blow up after all; we don't know why. That just felt disappointing to me.
I suppose there is a chance that the mystery of Makeb's continued survival might be explained on Imperial side (which I haven't played through yet). Alternatively, it might provide a good starting point to continue the storyline through dailies or other new content in upcoming patches, as we slowly unravel the planet's secrets.