Okay, I'll bite before I go play my Shepard. This is all just my opinion and by no means it is intended to be a blanket statement of the community as a whole.
What makes a "true" Mass Effect fan, in my opinion, is first and foremost someone that played at least the main numbered games and fell in love with them. Someone that likes RPGs as well as shooters, or at least likes one genre enough to try ME. This is someone that has expressed times where the plot in the series made them cry, made them think, made them laugh, made them sigh.
This is someone that will look at the series as a whole and look back at how they made them feel. For better and for worse, at times. I don't know if you have to be critical to be a "true fan" of anything, but I do know that a true ME fan tries to balance their criticisms with polite suggestion and feedback. Someone that is very passionate, at times to a fault, about what the story and setting meant to them. I have no problem with this sort of passion, so long as personal insults are left out of it. Shoot, I have Mark Meer's autograph and the very comfy N7 hoodie, and I've become passionate about the series as a newer fan. 
This is someone that will never forget what the impact those games were. Whether you still love the series, or if you've sworn it off after the pre-EC endings, these games had an impact on all of us. If it didn't, I highly doubt we'd be sitting on the official forums discussing and debating this series as a general community for the last 8-9 years since ME1's first E3 trailer.
That's just my take on it, at least. Now it's time to go do Noveria and mumble a lot of curses at the "black box bug." Thanks AsRock, you awful AMD processor you. I have yet to see Ilos as intended (I have to use a console workaround) but that will change when I eventually go back to Intel. I can't afford to for now, and if it isn't broke, I'm not replacing it. 