The reason the last two maps are great, IMO, is because they don't have any huge dominating terrain features like Citadel (River City), Mount Doom (Terra Therma), Caldera (Caustic), Lonely Mountain (Alpine), HPG satelite (Manifold) or the big mountain in Crimson Strait. Huge terrain features like that tend to force players to go left or right, most of the time. And it's either a circle dance, chasing each other around the center, or a peek-a-boo battle around the corners. With Terra Therma and Manifold, you sometimes get a full trainwreck in the center as well. But the matches are fairly predictable, there are no surprises.
With Mining Collective and Viridian Bog, the matches are a lot less predictable, because there's no huge terraine feature dictating your movement. There's dozens of places where the fighting can happen. I never get tired of those maps, really.
Not to mention, Viridian Bog is the best looking map in the game so far.
Spoiler
If you have some nice screenshots in High / Very High graphic settings, I'd love to see them
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