To me, Dominion represents a very interesting strategic shift on the part of Riot. It's faster paced, less teamwork dependant and overall a much "shallower" game mode than the 5v5 games on SR.
With the coming arrival of DOtA2, it feels like Riot is trying to create a niche for itself that will allow it to survive the potential flocking of players back to the "original" game of DOtA.
I don't know if this anecdote is truly representative, but I'll share it with you - I play LoL with my kids. My 13 year old son has been fascinated by the game and learning more and more about the complexities and strategies. We played Dominion last night, and after a couple of games he sat back and said "Wow, that was fun. Hey Dad, can we sign up for the DOtA 2 beta now?"