A lot of other people have said it quite well, but the Theramore scenario is a big failure in a lot of ways.
The scenario from the Horde point of view is as follows: You've gotten through the Alliance blockade and need to go rig powder kegs on the six Alliance ships in port. Once those are detonated kill the flight master and his gryphons. Then destroy the siege engines at the front gate. Now you find out that the horde had a spy here, who was imprisoned after his sabotage was discovered. Go kill the commander of the garrison and get the key to the spy's chains. Unlock the spy, who will port you out, while revealing that the entire siege and his sabotage was a feint. As you take the portal out you watch a huge mana bomb level Theramore.
Every time I ran the scenario I was more confused. Why was the Horde there? When had we decided to siege Theramore? Why were we bombing the shit out of them? Why were we using a Mana bomb? WTF could Garrosh possibly have been thinking???!?!
Garrosh
I'm really disappointed that the characterization of Garrosh has been totally upended. He had some interesting growth, and this seems to wreck all of that. When we first meet him in Nagrand he's a whiny mopey brat, too torn up about his father's failure to do anything constructive for his people. Thrall snaps him out of that, and he goes off to Northrend. There he gets strategy lessons from the elder Saurfang and a warning about honor. Time passes and in Cata he's in Stonetalon executing a general who *bombed* the Alliance. He even quotes what Saurfang told him about honor, and how it's more important that winning. Oh, and don't forget that in Ashenvale he's absolutely furious that an Orc demo driver uses some demonic energy to defend Splintertree from the Alliance. The only reason she and the player don't get killed is because you managed to root out and execute the warlock/demon that encouraged that corruption.
I'm not sure why Garrosh suddenly decided that his attacks in Ashenvale and Stonetalon were insufficient and he needed to piss of EVERYONE ELSE. I mean, with all the important NPCs who are now dead the entire rest of the world is mad at the Horde. Dragon flights angry? Check. Kirin Tor Angry? Check and double check. All the Outland factions that had a mana bomb used on them? Check. Alliance angry? They already hated the Horde, but the long voice of lets-not-start-a-war, was JAINA. The person whose city was just turned into a glowing crater.
I can understand the decision to attack Theramore. It is a strategically sound decision. Dustwallow Marsh has many resources and Theramore is too valuable a beachhead to let the Alliance continue attacking from there. The sack of Taurajo must be avenged. But a mana bomb? I'm totally confused as to why Garrosh chose that method of attack.
Game Continuity
This scenario was missing any sort of in-game explanation or reasoning at all. Both Horde and Alliance were just dropped into the middle of the siege and bombing. There wasn't even anything to tell you to look at the scenario UI to queue. Honestly I was expecting a set of quests to explain what was going on. It would have been totally logical to add in a couple quests for assisting the siege of Theramore and the creation of the mana bomb. If Blizzard had wanted to get complicated they could have put some extra phasing for a battle around Theramore and had some fighting in that phase.
Comparing Theramore to any other expansion lead in is almost too pathetic. BC had hundreds of demons pouring out of the Dark Portal. Viljo still fondly remembers slaughtering them, and still keeps the fabulous tabard from that event. Wrath had an undead plague and gazillions of zombies attacking the capital cities. Cata had the elemental invasion with elementals assaulting the world and especially the capital cities. For all three of those events and the Echo Isles/Gnomeregan event there was a really great limited time event. Yes, development time had to be spent on something that didn't last more than a week or three. However, the incredible unique experience of those events was not to be scoffed at. And I'm sure was a great subscriber boost too.
With the Mists expansion we've got a lack luster scenario that only makes any sort of sense if you've read the book, or surfed extensively online to figure out WTF just happened.
Edit: Oops got the date wrong on publishing this. It should have gone up a couple of days ago...