The Good: Smooth and responsive controls, using the bottom screen for a combo list makes playing easier, 3D effects are fun, lots of modes, utilizes Street and SpotPass very well, playing online is a blast, lots of things to unlock
The Bad: Fight system is convoluted, story mode is pretty average, could have used more fighters and stages
I have never been a huge fan of Dead or Alive outside the characters. The fighting system is convoluted, over complicated, and not ever explained very well. Dimensions breaks this barrier with a great Chronicle story mode that introduces these complicated techniques little by little so you can absorb it and it works great. The story mode is entertaining but nothing to write home about. You play as various characters trying to stop Alpha-152 which is Kasumi's evil clone. DOATEC is trying to use this to destroy the ninja clan Mugen Tenshin.
The fighting system actually is complicated but after the story mode you can actually use it. There are the usual combos of kick and punches, but you can juggle opponents and stun them in a critical state. You can throw and hold opponents while holding is kind of a counter attack you can actually counter a counter in this game. If someone counters your attack with a hold you can counter back or get out of the hold. Understanding how to do this take some getting used to but it adds some strategy to online fights. You can also do a super stun which is called OHing an opponent and this means they can't counter. There are also special stances that the characters go into which can be used to strategically add extra combos without the opponent knowing. You can also delay attacks in combos to kind of trick the other player in thinking your done so they stop blocking.
The fight system is fluid and smooth with great animations and controls, but my favorite part is that the combos are listed on the bottom screen so you can tap them or just use them as a quick combo list. This is really handy and actually makes playing the game more enjoyable so you aren't constantly pausing the game to remember a combo. On top of this you can now tag team which is fun and adds some great combos into the fray. I liked the extra modes such as tag team, and infinite mode, and of course the showcase mode where you unlock figurines to take pictures in 3D.
There are a lot of costumes to unlock to make the females look more sexy or the males look more cool and StreetPass and SpotPass are wonderfully integrated into the system. I'll have my 3DS in my pocket all day when going out then come home and have a few challenger ready to fight online that I passed when going out. I might even drive by a few SpotPass areas and come back with some downloaded costumes. Dimensions really utilizes these features and I have a blast using them.
With all that said the 3D effects are amazing and really show off the systems features in this way, but overall the graphics are smooth and crisp and look very detailed. I highly recommend Dimensions because it's the best fighting game on the system right now.