Connection
Although most of the time the game functioned without any lag, there were times where it would drop out completely.
I did like that it assigned a new host if the host left, rather than drop the connection completely, which would be great for RB6 Seige.
I felt the wait times to assemble a group were within reason, and in general transition times were not too long.

Game play
I though the mechanics were done well.The customizing of equipment was nice but very limited. I realize this was not the full game game but it was really repetitive playing dominion over and over. Even with increasing in level it didn't feel like the AI got any harder. Nor did the rewards increase. I saw it mentioned in another thread but people coming in at the last second and stealing your kills is really aggravating. If I do 95% of the work getting a hero down and someone comes in and take the final blow and they get the credit for it there becomes little incentive to actually fight and rather just wait for other people to do the work. I had issues where I was the only real player on a team of 4 with them not even trying to revive me.

Maps

Some maps felt too easy with the ability to just push heroes off a ledge. That really just takes away from actually having to combat enemies.