More than a re-skin, Monster Kingdom will change your playstyle but still feel like the Majesty you love (or hate).
Monsters are hardier than their human counterparts, and for good reason - it's hard to buy a healer worth having, especially in quantity. If you like to turtle you can still do it, but this expansion really favors aggressive play. The mix of units isn't exactly turned on its head - you can make some comparisons without too much difficulty, for example saying a minotaur is your "dwarf." Except that other than the price and load of hps, its abilities are a mix of fighter and blademaster. Changes like this are everywhere, and make for a pleasing new blend of hero and party dynamics across the board.
The campaign is well done, with 10 varied and often creative missions. The narrator returns as before, doing a fantastic job delivering the story. Which is good, because the translation is one of the worst I've seen in a finished product. Typos abound, and after a while I stopped reading and just let my ears finish the dialog for me (and I am usually the kind of person who reads the text three times faster and skips the voice-over halfway through).
But back to the campaign. You have missions to unlock units, survive assaults, and complete timed objectives - as before - and you also have some creative twists in them, such as a magical inn that randomly appears on the map and regurgitates parties of enemy heroes. Campaign scenarios start at advanced difficulty and continue to expert, though your mileage may vary. Some of the advanced were hard for me (but it could be simply due to getting used to my new faction) and some of the expert were easy, but if you're a serious majesty player there is a lot to sink your teeth into.
Finally, the game does have a few problems that take away some of its shine. The translation is a major knock against it, and I did encounter a few bugs. One scenario had ogres turning invisible at times, and in general the liches have a confusing stat build. Their abilities say they use magic power, but I've had liches created with 1 magic power and 17 strength, so something is not right.
The biggest problem though is the lack of any single-missions that allow you to play the monster faction in all its glory. In fact, the game ships with zero new missions, only 4 more DLC missions in the shop (all of them human faction). It's a shame fans will have to pummel through the campaign to finally reach a point where you can play around with everything without a timed objective or vicious, recycling monster rush.
But, if you're a majesty 2 fan, I'd advice you to pummel away in spite of the issues. Watching a high level lich mow down a group of heroes with a pestilence cloud (that also stuns and slows them) is a beautiful thing.