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!
"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."