At it's most basic levels it fails to succeed in any of the places that earned it's predecessors so much acclaim.
Mass Effect 3 depended on it's faithful following to run out and buy their copy day-of-release, unfortunately it hasn't quite worked out like that. Within hours of play many purchasers immediately flocked to the messageboards to vent their frustrations at what the developers failed to do on the most basic level. For instance: as soon as one loads up the game and prepares to import their "Commander Shepard" from his frolics in Mass Effect 2 they are greeted by a great big slap in the face. The "Face Import" issue stands as just one of many issues which directly penalise old-faithful players, and once you have an error and the immediate let down 30 seconds into the game it's hard to recover.
Yes, you can reconstruct your Shepards appearance, but the human mind is programmed to recognise faces, so there will be many moments where micro-movements caused you to realise that this is not "your Shepard".
The game mechanics are just as clunky as they have always been (yes there will be moments when Shepard will 'click' onto a flat surface and ignore your command of running forward away from the massing legions of deep space, leaving him to be pelted by bullets while the camera whizzes around like it's mounted to a drunk guy on a skateboard). There is a vast array of superior third-person shooters out there on the market. But this issue has always been overlooked in previous instalments, because of it's stellar story.
Unfortunately, Mass Effect 3 completely fails to deliver in it's most important selling point. I don't need to tell you that after your investment of nearly 100 or so hours (spanned out across all three games), the ending, which is so important to any videogame, goes so far as to ruin the entire series, to the point where you probably will not want to play any of the previous games again.
Mass Effect deserved better, and whether it was just down to lazy-writers or too much publisher interference, Bioware have let themselves down spectacularly. (An incredibly hyped game from a groundbreaking series, doesn't have to have an underwhelming conclusion, as demonstrated by 'Metal Gear Solid: 4')
With any other name, Mass Effect 3 would probably be confined to the nearest discount section of your store. It's a shame to see such a series bow out in a whimper and leave nothing but a trail of discontent and disappointment in it's wake.