I disagree with your second to last "con"SpoilerI think it have a lot of meaning to the reapers motives and made it more tangible.
SpoilerI don't see a problem with the synthetics vs. organics conflict. The Leviathan DLC underscores the fact that the reaper AI (catalyst) was seeded with a false assumption, thus all their subsequent conclusions would be flawed.
Like I said, I preferred knowing little about the Reapers. All I knew is that they had to be stopped. Synthetics versus organics was never as major of a theme as it is in the last ten minutes of the game or this DLC, which is why I'm still not sold on it. A DLC shouldn't have to explain "this is what we were going for all along!" That just shows, in my eyes, how badly they missed the mark in the actual game. Honestly, though, after the original endings and all of the fallout, I'm done wasting energy on being angry or disappointed. What we see is what we get and at least it makes a little more sense now. I'm just glad the content was enjoyable to play through.
I actually kind of liked the way they left it,Spoilerwith the Leviathans refusing to join the main battle but agreeing to waste any Reapers that entered their local area. Maybe mostly because it reminded me of Arthur C. Clarke ("All these worlds are yours, except Europa").
I think an extra slide at the end of the game showing a Leviathan starting to emerge from the depths would have been fine. That's all that would be needed to keep their ominous nature in tact and get the speculations flowing.