Speaking as a non-Sims fan, I found The Sims Medieval to be thoroughly enjoyable and surprisingly inventive.
That said, players who found the main series' complete lack of direction to be frustrating can finally understand what all the fuss is about. In Medieval, the player is The Watcher, a powerful being who desires to bring order to the hapless sims' lives. This isn't simply fluff either; the sims actually worship the player and the game features two different denominations which the player can experiment with. The player controls his sims only during quests. Each quest is a short scenario with clearly stated goals which provide rewards based on how well the participating sims perform. In true medieval fashion, each sim is locked into a specific job during the creation process, and some quests can change depending on the professions of the player's party. For example, a Jacoban priest may respond to flagging faith by recruiting the local knight into an inquisition, while a Peteran priest may redouble efforts to bring new sims to the fold.
Completing quests earns your sims money and experience--which contribute to increasing performance in their jobs--earns you resource points (RP), and improves the kingdom's aspects. RP can be spent on new buildings, which increase aspect limits and can bring new professions to the kingdom. The aspects are a strategic component of the game which represent the kingdom's overall status regarding culture, health, security, and intelligence. These aspects have negative consequences if ignored. Low security, for example, results in large numbers of bandits plaguing the kingdom, while low health can cause rampant disease. It's not terribly hard to do well on the quests, as obtaining a good performance is mostly a matter of keeping the active sims focused long enough for the quest meter to fill, and keeping them focused simply requires taking care of needs, doing things they enjoy, and compensating for their Fatal Flaws (negative attributes selected during sim creation). This means that quests with multiple sims are, contrary to what you might expect, easier to do well on, as two focused sims can reach the highest performance level more quickly than a lone sim.
All of this combines to create a surprisingly endearing portrait of a quirky kingdom, and the rotating character control turns the game into a varied, television series-like story of the trials and travails of a community. It becomes very easy to feel like your sims have actual personalities when the game provides so much context for how they behave. Why, of course my Evil Jacoban priestess would start an inquisition! And of course, after being converted to the Jacoban faith, my Bloodthirsty knight would choose to execute the heretics rather than exile them! That's exactly how I would expect them to act based on their previous behavior!
The game's major weakness is that the most interesting tasks in the quests, such as sneaking into foreign territory to steal a relic or doing battle with a dire chinchilla, are presented solely as text boxes. While not a huge problem during a first playthrough, starting a new kingdom and trying different approaches shows how the quests are mechanically very similar, and repetition suddenly becomes a significant issue.
It also has to be said that the game does not allow any customized building layouts. If you love The Sims as an architectural exercise, you will be sorely disappointed here. However, clever use of Furnish Mode allows the player to considerably change the feel of the buildings, so there is still a great deal of customization to be had.
In the end, The Sims Medieval is a game well-suited to anyone who felt they could enjoy the mechanics of The Sims if only it was a little more structured. Long-time fans may be alienated by the more regimented structure, but for everyone else there's a great deal of inventiveness and charm to be had here. With an expansion or two to iron out the rough spots, this could become not just a quirky offshoot but a satisfying and full-fledged series in its own right.