[quote=Ariamella]
And I'm honestly curious- because I've never considered it before- but how is the seed of doubt and mistrust more damaging to an online relationship versus an offline relationship?
[/quote]

You can't check in on a LDR like you can with one that's a lot closer. You can't do things to confirm where they've been, who've they've seen, if they're going where they say they're going. This isn't to say that you should stalk your significant other, or make them tell you where they're going every time they sign off Skype, but those seeds of doubt can't be removed like an offline relationship. When you notice your LDR partner getting more and more distant, not logging on when they usually do, being more vague than they used to be... There will be doubt and mistrust. Think of how shady the internet can be. People have mentioned catfishing as one example. If someone is cheating, there's no way to know unless one party lets it slip, or decides to come clean. 

I know LDRs can work. I've seen them work.

I've also seen them damage people beyond immediate repair. 

Essentially, LDRs are pretty much like offline relationships in most ways.

I will echo everyone who says LDRs require more communication, though. You never get to see the person that you really like spending time with go about their daily activities. You want to get to know them. You want to feel close to them. Sometimes sitting in mutual, comfortable silence can be incredibly soothing. Those who are unwilling to communicate in a LDR typically doesn't do well in a LDR.

Then again, this is all just my experience, so feel free to disregard, dispute, and disagree with everything I just said.