I think the Paragon/Renegade system has two massive faults, one subjective and one objective.
Subjectively, it's consequenceless auto win. Which to me is just a bit boring when it beats at the heart of a "role playing game", where difficult choices with no perfect outcome are far more intriguing. Frequently, at least from ME2 onwards, Paragon/Renegade choices give you the best possible outcome which, to me, makes dialogue choices in general pretty redundant.
Objectively, I think the way they're designed is such that it's extremely easy to create a domino effect of Paragon/Renegade points where you really don't have to work towards future options. It only becomes a problem if you're playing a mix of Paragon/Renegade, and I figure most players didn't. Save file import combined with pure Paragon or Renegade borderline guaranteed you'd always have enough points for every Paragon/Renegade scenario, earning even more points as you'd continue with this path of role playing.
I know that BioWare isn't really interested in mutually exclusive game states and prefer players see and experience as much content as possible. And the linearity of the series prevents it from being truly branching. But for both my Paragon and Renegade characters the dialogue system became redundant. It was always an option, never locked off, and always the best choice for that arc. Ergo, boring.
To be fair though measurements of good/evil in video games are always a bit clumsy, especially so in games as linear and lite in content diversity as Mass Effect. If they absolutely had to have something I'd prefer less arbitrary Paragon/Renegade and more simply notoriety.