Okay first off I have to admit that I'm not a fan of The Sims 3, I found the game quite uninteresting honestly and not very involving but there's something about The Sims Medieval that makes it stand out as its own in term of game-play and frankly the whole thing about you being The Watcher and your relation with your heroes and the people inhabiting your kingdom to me feels kind of like mythical.
When I play this game it's like I open up a book that lets me live in my own King's Quest fantasy kingdom and for once the simmerish talk that the sims speak is more relevant here than in The Sims 3. It feels more natural as you can pretend they speak a medieval language that you can't understand unlike in previous Sims game where you would hope they would speak English.
So basically The Sims Medieval went out of the formulaic open world of the previous Sims and built upon a more structured scenario with lots of quests that you can play with different heroes and how you'll solve those quests is entirely up to you, yes for once you can feel like role-playing with a good element of choice in place for your enjoyment and to me, though the game has flaws and is prone to repetition, is quite a feature and gives the game its charm and makes it stand out as unique.
You may not like this game but you can't disagree that it is...
Unique.