I thought I'd post my feedback. I got to play somewhere between 15 - 20 games of Dominion in the last couple of days.
I love it. It's a lot of fun.
Things I feel are really good:
1) Fast paced, frantic nature
Summoner's Rift can be a little slow at times. I like the fact the games are shorter. I've had games as short as 10 minutes. Yesterday my wife asked me to go for a walk for her when I was playing Dominion, and I said I'd be between 5 and 10 minutes. (This was 14 minutes in). The game was over in 3. On Summoner's Rift, I've always had to say "I don't know how long I'll be".
2) Passive XP and Gold gain
Helps keep players who may perform poorly at the start competitive. Sometimes on SR you just can't catch up, especially with certain Champs if you get a bad lane match up.
3) PvP intense action
I like that overall, the minions and towers have less of an overall influence on play. You feel a lot more free to move about the map.
4) New unique items
I love the creativity put into a lot of the new items.
Some things that I found concerning and might need improvement.
1) Rammus is stupidly powerful.
Every game I picked Rammus I won. I started picking other Champs because it was too easy as him. He can make it to the 12 o'clock capture point straight from the start in time (with Boots of Mobility, Ghost, Powerball, hit the MS shrine on the way) to capture it completely before an opponent can be there to contest it. It makes it a lot easier to defend that point once he's captured that.
Not only that, but he is one of the most durable champions available. Even building him mainly for capturing with MS and Pricilla's, he is still really hard to kill. Not only that, but he also becomes an excellent chaser with Tremors to finish off low life Champs.
2) Move Speed builds seem much stronger than any others.
One of the first games I played with Warrick I built a DPS style, which is my normal style. It failed miserably. A few games later, I tried him again, but built him mainly for MS (Force of Nature, Boots of Mobility, Phantom Dancer). Result: 515 MS without his Blood Scent ability activating. I did the same thing with Nocturne and had even better results: Noc seems pretty strong.
I feel for Dominion, % MS items are perhaps too strong, especially since they are already great items in other regards. Maybe Dominion should have low flat MS versions instead of % versions?
3) Certain Champions are ridiculously powerful or annoying
Rammus, Alkali are two I find often mentioned. I found Singed really annoying as well, because you just can't kill him without 2-3 champs hitting him at once. Which means he can constantly come back in, disrupt your capture without fear and then run away, leaving his purple poison trail behind him.
4) Defending points is really, really hard, and not worth it unless there's a Quest buff.
I found it of little benefit normally to try and hold on to a point unless my team was leading 4-1. Normally it was always easier to zip out, try and capture another point while the other team was piling up on some poor sap who was trying to defend the bottom.
I feel that part of this might be the tower range. It seems really short. While it's nice that Champions can neutralize towers alone without any assistance, it also makes defending a point very difficult, because generally the enemy can harass you without taking any damage from the tower.
5) Another concern I heard was that AP champs are underpowered. I can potentially see why. The Dominion Aura buff I think grants more Armour Penetration than Magic Penetration. I know that giving bonus AP won't necessarily give casters an edge as it will buff everyone else considerably. I think casters have problems in Dominion because:
i) They're generally slow - bad for Dominion
ii) Some of them have long cool down abilities, which means once they use their skills they have to run and hide until their cooldown is up.
Anyways, that's my feedback. I hope it is helpful to you in making an awesome game mode even more awesome.