Definitely Not Dominion is poorly designed and not fun.
Essentially, here's what happens:
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There's a fight at the top turret.
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Whoever wins that fight wins the game.
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The game drags on for another ~15 minutes.
The basic problem with DND is that there is nothing to "trade" for the top turret. The other turrets are too close to base, the mid buff does almost nothing, and the super-minion is "yawn". There is no sense of "they take this objective, but we can take this one." A team with top control has no need to fight for any other objective except the easily defended turret near their base.
I have played this mode every time it's been up, with a variety of champion choices, organization levels (5 voice-com friends vs. Solo Q), strategies, etc. etc. etc. I have seen a very small amount of close games, but those games are usually intentional clown fiestas where neither side cares about victory and just wants to rolfstomp ham. (I am okay with that in RGM, but even for that, most other game moes are better.) In any game where teams are trying the slightest, the outcome is the same about 95% of the time:
Whoever takes top first and makes the most basic attempt to keep it will win the game. And most games drag on for way too long with no chance of a comeback. I am not someone who plays normal summoners Rift and thinks "Well the other team got an early lead so let's quit." I like to be able to fight back, and even if I lose I don't get upset because I always felt there was a chance to outplay right until the end. Whether i like or dislike the flavor or style of other game modes, I see frequent close matches in them, or at least I can enjoy them despite getting beaten, because I feel there is a chance.
tl;dr - DND victory is decided in the first two minutes, then drags on forever and is boring.
This is poor game design.