In short, I'm not particularly keen, but let me expand, as I don't mind this to be mindless QQ.
The crux of my argument revolves around Twisted Treeline. Twisted Treeline is the short, fast paced tanky dps map. Maybe not as fast paced as what Dominion will be, but nevertheless, I believe this feeling will be captured in a similar way.
Now, I really hate Twisted Treeline because of these points. I focus my game around strong lane dominance, yet I, and I imagine many others, feel that most of the truely good and interesting play comes in late game teamfights, which obviously won't occur on these two. Not only that, I don't want to play tanky dps. I hate playing them, I hate facing them, and the thought of 2 tanky dps very slowly killing each other camping on a control node doesn't get me too excited.
I don't think it's inherently wrong that you've tried to make a new mode, in fact I take that back, it's worth applauding. However consider this. It is obvious Dominion is much more comparable to Twisted Treeline than Summoner's Rift. It is also obvious that Summoner's Rift is vastly the more popular map. So, why would you make one that's more like the less popular one?
The best way I can think of phrasing it is this. Right now, the general reaction is a few people happy, most people 'meh', a few people very unhappy. Imagine the reaction you would have got if you had announced the imminent release of Magma Chamber. Seriously, don't be biased, imagine the genuine response.
That is the point I wished to make. Your expanding the game making it stretch, thin and shallow rather than deeper. Build on what you have, then by all means think about expanding into new modes.
Lastly, it is very clear Dominion is intended to be a fun, mess aroundy kind of mode. This obviously means if you want to play competitively, or play at any kind of high level, you will be playing a hell of a lot more Summoner's Rift for a very long time. Not cool.
EDIT: people seem to be thinking I'm making assumptions from the Riot trailer, which I'm not. If you go around various gamesites like gamespot or PC gamer you'll find many interviews with Riot where they go into vastly more detail. That's where I'm getting my info from.