In a way, this question is a little unfair. True, I didn't love the Mass Effect games nearly as much as the Dragon Age series, but then it's rare that I've fallen as completely in love with a game as I did Dragon Age: Origins. So that's a high bar to set it up against.
Also, I did like Mass Effect, quite a lot. I enjoyed playing all three games, found strong points and favorite missions and characters in each. Commander Shepard, as played by Jennifer Hale anyway, is one of my favorite protagonists of all time. (I've never played as m!Shep, so I can't comment on that interpretation of the character.) The series was worth playing, and I might pick it up again someday for a Renegade run, and I still need to replay endgame to see the Extended Cut ending. But I didn't connect with it fannishly at all, and it's never going to rise to the level of most beloved game.
I can't point to any one reason for this, although I can speculate about various factors. First and possibly foremost, the character interaction felt lacking in many ways. This might be a case of suffering in comparison to Dragon Age, which has such strong characters and such great interactions, both between the PC and other characters and the banter among party members. In Dragon Age, especially, but also in my favorite FF games, I was able to really get to know the other party characters. This seemed lacking in ME, especially with the lack of party banter. I felt like I saw sketches of great characters and relationships, but there wasn't enough detail for me. They lacked the shading that was really needed to round everything out. Especially the romances -- I romanced Kaidan in ME1 and Garrus in ME2 and ME3, and although I liked them well enough, they struck me as shallow in comparison to the Dragon Age romances, especially Kaidan's. (In a way, the most interesting thing about either relationship was the meeting with Kaidan on Horizon in ME2.)
This issue was particularly pronounced for me in Mass Effect 2. The structure and pacing of that game didn't work well for me. So much of the gameplay was centered around triggering and completing loyalty missions, and with such a large team to juggle, it took a lot of effort to make it all work. And even then, I didn't get the final conversation with all my teammates - some of them were still saying "can we talk later?" even up to right before the final battle. That made it harder to really connect with them, and it was also frustrating -- if you won't even talk to me, why did I go to so much trouble to help you? It also served as a distraction from the main plot, to the point that it started to feel secondary at times. Of course, all RPGs can possibly suffer from sidequest syndrome, but I found it more pronounced in ME2 than average.
It probably didn't help that I had difficulties with the gameplay. I am mediocre at best with shooters, and trying to manage combat while driving the Mako or the Hammerhead was totally beyond me. Even with all the assists turned on, and sometimes turning down the difficulty to casual, I often had trouble with it. I played the games with T, so I was able to hand the controller over to him when it got too frustrating (and he did almost all the Mako missions, which worked out because it was his favorite -- I bet he would play a whole game that was nothing but driving the Mako around planets and shooting geth), but even still, some of the battles were ridiculously difficult. Particularly in ME3, where anything with more than one banshee at a time involved a lot of dying, even if we switched up the difficulty. Dragon Age had its tough battles for sure, but nothing that wanted to make me throw the controller at the screen quite as much as those.
All that said, who really knows why I connect with one game, one fictional universe, and not another? Sometimes I can say exactly why, but other times it's more nebulous. It's also entirely possible that Bioware fandom set my expectations for Mass Effect much too high. Maybe if I'd played the ME games first, I'd like them better, or if I'd spread them out over the years rather than binging on the whole series within a few months. In the end, I'm glad I played them, and that's always enough to hope for.
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