Mass Effect 1 did do the best in exploration. So many different planets to explore and catalog, I didn't even mind that most dungeons were basically carbon copies.
I wasn't the biggest fan of the gameplay for one reason: it was not well-differentiated. In 2 and 3, the different classes played VERY differently, whereas in 1, a Vanguard was basically a gimped soldier with some Adept moves and stuff instead of being a Charging, Shockwaving, Novaing beast, an Infiltrator was a shitty Engineer with some Soldier abilities instead of being a cloaked assassin, a Sentinel was basically a shitty everything instead of an invincible wall, and so on. They REALLY polished up the gameplay in 2 and 3 which is a large part of why I enjoy the gameplay in them so much.
I had two thoughts about the plot threads it introduced.
1) Entirely made to attempt to justify their ending.
2) Fucking ridiculous.
So the Leviathans noticed that all the species they enthralled were creating AIs that rebelled... so to counteract this... they created an AI. Which rebelled. Face fucking palm.
To be fair, the AI they created was supposed to be a peaceful one that fostered understanding but ended up fucking them over - in the end it did end up fulfilling its purpose. Honestly, I still don't think they need to 'justify' anything - they already have the extended ending for that. I found the reveals fine. I mean, nothing we didn't know already, but hey, interesting exposition, nonetheless. I mean, I prefer this to Omega DLC, and I REALLY WANT Omega DLC.
I just wish we got to see the Leviathan of Dis. :|