The characters.This will likely be controversial, but why is Citadel DLC so popular? I’ve never played it, but from seeing the sheer overload of cringe marvel humour in it, I never intend to.
It doesn’t feel like a serious story at all.
The big appeal of the Mass Effect series, (and BioWare RPGs in general) is their cast of interesting, entertaining characters that you get to know in the process of bringing them on missions and chatting with them back on the Normandy. And with the ME trilogy, many of these characters are ones you've gotten to know over multiple games. As someone who played the ME games as they originally released, they felt like characters I had literally known for years.
So Citadel is a chance to "let your hair down" with all those characters. To just have fun. You get to have one-on-one time with each of them. Then you throw a huge party and see all these characters interacting and getting up to hijinks. If the characters were a highlight of the games for you, I struggle to think why you wouldn't enjoy the opportunity to hang out with them like this. Yeah, it doesn't really cleanly fit in the story of ME3. But whatever. This is one case where messy continuity doesn't really bother me when the result was this enjoyable.
Other points: the fan inside jokes. Oh, the fan inside jokes.
"Shepard. Wrex."
"*I* should go. I *should* go."
"It just tastes like sweat. Why would you even ask that?"
The actual mission of the DLC is just fun, lightweight stuff. Again, it's nice to see your whole squad interacting. And a big plus is that it brings back fan favorite Wrex as a squad mate (if he's alive).
The arcade mode that brings back every past squad mate from ME2, as long as they're alive, is also quite fun.
And lastly, it pulls at your heartstrings as a final sendoff with these characters. I mean, if you don't feel anything in this final scene (with your preferred love interest), I gotta wonder how much you enjoy the Mass Effect games to begin with.