Does paragon/renegade/neutral morality affect the storyline or available missions?
Will focusing primarily on paragon or renegade (or neutral) affect the storyline in any major way (IIRC, in previous games it was mostly just dialogue options)? Will I miss out on anything, such as items or missions, if I do this?
Apparently, the final dialogue option of the game depends on your Paragon/Renegade score, and not just your overall reputation. And it does so in a rather strict way - you must have either gone 100% Paragon or 100% Renegade to unlock that option.
It doesn't make a huge difference to the story though, and every other option only depends on overall reputation, so it's not that big of a deal.
Short answer is no. There’s no penalty for mixing Paragon and Renegade
Your paragon/renegade/neutral morality does not affect story line or available missions. All your paragon, renegade and neutral points are added together to make up your total reputation. Special dialogue/decision options may become available when your reputation increases, regardless of whether you did or chose more renegade or more paragon actions/dialogue options.
Read this blog entry - it explains everything!
Excerpt:
"On your squad/powers screen, you’ll see a bar made up of a mix of red and blue. The red represents your Renegade points, while the blue represents your Paragon points. Reputation points that aren’t Paragon or Renegade don’t get their own color – they make the bar get bigger while keeping the same red/blue ratio.
Note that the bar has lines marking various points of progression. Key dialog options at important moments in the game are locked off – you can only take them if your reputation is high enough – and each of those lines marks a checkpoint. If you see that you’re a bit short of hitting a new line, and someone has just said something like, “Let’s head down to [that person's homeworld] and finish this once and for all,” it may be worth your time to go do a couple of side-quests first, just to see if you can reach that line."