The fact that you can cancel any attack into a tag provides amazing options, especially for characters who don't have a lot of combo potential by themselves (Ichirin is a good example).
Creates some really fast-paced fights too.
Marisa + Reisen is a very interesting set of characters because Marisa's neutral special (the bomb) combos into Reisen's projectiles, giving a good mix-up for a combo starter. In fact, these two characters alone can give an interesting edge to characters with not a whole lot of a projectile game (Mokou was interesting in the demo, but I'm looking squarely at Byakuren).
Personally I'm looking forward to whoever manages to make Koishi tag combos...I will have their utmost respect.