It's not just plot which drives it in this direction - Obsidian has integrated a mechanism which introduces itself to the game in the second act and increases its sense of character. It's - well - I don't really want to spoil it, as the reveal is pretty neat, but imagine if that bit in Oblivion where you become a vampire wasn't incredibly annoying. It's like that. Clearly, having to worry about something other than where the next dungeon is will annoy certain direct-minded players, but it's one of many things which make MoTB feel like its own creature.
In fact, since it is a leap up from NWN2, you're tempted to suggest people just jump straight in. Problem is, you'll be really hammered by it. You start with level-18 characters. That's a mass of spells. Building a character from scratch will be hard work for those who can't sing the Monster Manual backwards, and even if you go for a pre-generated character you'll be left trying to familiarise yourself with a lot of options very quickly. Manually generate a character and you'll easily make someone who isn't much use. Hell, I chose a pre-generated Favoured Soul with some neat monk-y abilities from the Sacred Fist class. I figured that a Cleric with some fighting abilities would be hard to go wrong. Except that I didn't know the enemies at the start were spirits, most of whom wouldn't even be able to be touched by my character's fists. It's high-level D&D, so you do need to know what you're getting into. Even turning the difficulty down to Easy doesn't exactly make progress, er, easy.
There are also some changes to the engine, which, while they aren't exactly problems, do alter the way you play it. Resting to recover spells is a regular thing in any D&D-originated game. NWN2 chose to break from the rules a little, and have you resting for five seconds. However, MoTB returns to the purist situation where you rest for eight hours in-game-time to recover, with the chance of wandering monsters or whatever. While discouraging you from resting - a good thing - the idea of staying down a dungeon for eight hours remains one of the sillier parts of the system and a real atmosphere-breaker.
Also, the game's a little showier with its XP status messages. What was previously lost in the message box is now flashed on the screen, and this can be fun when you get an XP bonus for an unusual act. Being very strongly informed whenever you gain or lose influence with one of your party members pushes you towards actively gaming them - treating them less as characters and more gauges you're trying to fill up. "-6 INFLUENCE WITH WIZARD LADY!" makes you consider reaching for the quick-load in ways that another approach would not.
A minor problem is that while you're enormously powerful you don't exactly feel like it. You're an Epic-level adventurer, and you're still being treated as if you're a kid from the Swamp. People should be impressed by you while you're messing around with Gods and similar, but they're not. It's a perennial problem in high-level adventures on computers, but it being commonplace doesn't make it any less grating.
There are other issues in the plot, too. Take your companions, for example. While I quite like most of them, they're an enormously serious bunch. You suspect Obsidian realised that the Crazy Mob of NWN2 was a little too much, but now we've gone from being a member of Madness to being a member of Joy Division (to go for an inappropriate post-punk metaphor). It's not to say there's not humour in it - there's a scene involving Frost Giants which made me laugh harder than any videogame since SumoTori (and, yes, even Portal). But the timbre is a long way from Dwarf Warriors wanting to become monks because they're good at fighting. Oh, and while the actual campaign itself is polished enough, there's a nasty bug which breaks the original campaign unless you install it in a very specific way. Frankly, Obsidian and Atari need to sort this out.
So where does that leave us? A mass of excellent content - any add-on pack good enough to make you start the original has more than a certain something - with a few problems. It really isn't for anyone other than the devoted western-RPG head. Which is fine; the devoted western-RPG head has had a particularly weak year, and will lap this up. As they should. But if you're not in their ranks there's little here for you.