A solid RPG with the quirkiness of the Mario universe.
The Thousand Year Door is a fairly hardcore RPG, it's got a fair amount of side quests, a fairly lengthy story with lots of text and fairly slow moving turn based battles. These things can be seen as a positive or negative point depending on what type of gaming you're into. If you are huge into old-school style RPG's this is definitely up your alley, if you want something with a quicker pace then stray away.
The battles do add something to the typical turn based formula though that I was perticularly a fan of, that is the ability to interact when you are being attacked. With a perfectly time button press you can block/dodge enemy attacks, what you do depends on what type of enemy is attacking you. For example, something that is going to attack you from above or with a weapon you will want to block, but a turtle that is going to slide at you you'll want to jump. Through some leveling up you'll even be able to deal back damage when getting attacked.
When attacking you can also do some timed button pressing to deal some extra damage. These button presses change depending on how you decide to attack. The two basic forms of attacks are jumping on enemies or hitting them with a hammer, this will depend once again on what type of enemy you are fighting. You can also hit enemies with your hammer while on the field to initiate a first strike bonus attack (battles are not random, enemies are visible on fields).
The dungeons you'll trek through all have special characters inside that you'll meet up with to help you solve the puzzles tricks and traps of which there are plenty. It will actually require a decent amount of thought to get through most of these dungeons which may come as a surprise when thinking about the normal casual difficulty of Mario games, but as I've said, these are some pretty hardcore RPG's. The story is moderately interesting although it doesn't quite come up to par with many RPG's out now days, but you'll get to read plenty of dialogue between plenty of characters throughout your journey here which can become painful at times. The graphics and sound are of good quality. If you've seen any of the Paper Mario games you know that they have a distinct graphic style in which everything literally resembles paper, a very cool concept and a unique one at that featuring a lot of cool tricks throughout the game and also provides some good puzzle situations.
That pretty much sums up The Thousand Year Door, a solid, but lengthy and slow paced RPG. If that's what you're into you owe it to yourself to play this game, but if you like to move along at a quicker pace then stay away as you won't find what you want here.