Okamiden is bit too much like its predecessor, but it provides good fun and cuteness galore.
The story, at least initially, bears a lot of resemblance to the original Okami. When I say the world is plunged into darkness again, I mean that quite literally - Chibiterasu must reprise his mother's role in making the guardian saplings in the world bloom once more in order to banish evil from the lands and release the people of the land from darkness. Along the course of his journey, he'll team up with several different partners, visit lands both familiar and new, and ultimately confront a new ultimate evil.
The story in Okamiden is significantly more character-driven than it was in Okami. Characters in Okami were more or less one-dimensional and did not experience any significant character development (with a few exceptions - e.g., Issun), whereas Okamiden fleshes out quite a lot of things from Okami and develops its new characters quite a bit more than Okami. A few things in Okami are explained in Okamiden in a rather clever fashion, as well, such as how the Goryeo sank and why the magic mallet was present in its cargo, which is something that I quite enjoyed seeing. Overall, I felt the story in Okamiden was improved a fair bit over Okami - but, even so, it still was not really the main attraction. I will say, though, that it got significantly better as the game progressed, which was good to see.
The gameplay in Okamiden is very similar to that in Okami. As Chibiterasu, you travel across Nippon, interacting with the denizens therein and solving their problems with brush powers - abilities that have you draw on a canvas overlain on top of your view of the world, and which create different effects within that world depending on the shape drawn on the canvas. For example, drawing a straight line on the canvas will slice through anything the line intersects, while drawing a bomb shape (a circle plus a line) will cause a bomb to magically appear in the field. You also encounter enemies along the way, which trigger entry into a battle scene, in which Chibi can either hack and slash away at the enemy with his Divine Instruments (mirrors, beads, or swords), or use his brush skills to attack them. Chibi can't use unlimited brush skills, though - they drain his ink pots, which can be refilled by either using inventory items or by picking up ink pots from breaking jars in the field - so brush skills shouldn't be just spammed like crazy. One very notable difference between Okami and Okamiden as well is that the ink pots don't refill automatically anymore, so using them sparingly is even more important.
Those who have played Okami will probably remember the praise system from that game - restoring trees and cursed areas and helping out creatures generated praise for Amaterasu, which could then be spent to augment Amaterasu's abilities (e.g., increasing her health and ink pot capacity, increasing her wallet size, and so forth). That system is back in Okamiden, but it's been quite considerably streamlined from the first - now, instead of spending praise, the game just has a "praise meter" that fills up as Chibi gets praise, and when it gets completely full, Chibi automatically gets either another unit of health (or solar energy) or ink pot. The other upgrades from Okami are basically already in effect - as far as I can tell, the wallet Chibi starts with is already bottomless - so this wasn't a big deal, although I do have to admit I liked the freedom to choose how to upgrade from the first game.
Much as in the original Okami, Chibi acquires his brush skills from brush gods - deities that are much like Chibi in appearance - white animals with red markings. Even if the player already knows what a brush skill looks like, it's not until Chibi actually meets with the brush god that that skill can actually be used. All of the brush skills from Okami reprise their roles, along with two new brush skills as well, the most significant of which being the ability to control one's partner - if Chibi's with a partner, he can have that partner dismount and can then make that partner walk in the field by drawing a line from that partner to where he'd like him or her to go. I have to admit that I was actually a bit disappointed by what I felt was an overreliance on gameplay from Okami here, though - rediscovering all of the brush skills that we had already seen from Okami was lacking in the mystery and intrigue that the original had when each brush skill was new and fresh. Still, it manages to keep it interesting by having Chibi learn the brush skills not from the original brush gods from Okami, but by their children - children which are, much like Chibi, SO ****ING CUTE.
The music and graphics are also definitely in full effect from the original, too. The leap from the Wii to the DS definitely incurred some logistical hurdles - the distance you can go between having to load new areas is much, much smaller, and Shinshu field (for example) now has to be divided into three parts instead of being one big expanse - but the style heavily infused with traditional Japanese art is still very much intact both aurally and visually, and is very much still the treat that it was in the original Okami.
All in all, Okamiden's biggest flaw is, in my opinion, the fact that it borrows too much from its predecessor - the vast majority of the brush skills you acquire are those already seen in the first one; the gameplay is quite similar to the first one; the premise is quite similar to the first one; about half of the settings in the game are from the first one; and overall the game feels as though it could have differentiated itself from its predecessor much more than it did, and could have further built upon the world of Nippon much more than it did. Still, though, taken unto itself, its story is good; its gameplay is solid (the brush skills feel more natural than ever before on the DS); it's a treat for the senses; and it's an all-around good time. If you enjoyed Okami and wanted more, you should do yourself a favor and check it out. If you didn't enjoy Okami, on the other hand, it's not going to offer anything more to win you over. If you haven't played Okami, then it'd probably be best to start there - this game is a direct sequel, after all, and a lot of things in it will be better understood by those who have played the original.