I didn't love Riley, not my type. But I liked him for Buffy (even though I didn't see it lasting forever, it was a nice thing for her). And even though I thought it was rushed that he was all of a sudden leaving forever if she couldn't immediately "get over" what she'd just learned, I did feel for her when she ran and he didn't look down.
I sorta felt like he purposely didn't look down, I mean, if he really hoped she'd show...he'd have kept watching till the area was out of sight. I felt like he'd made up his mind.
I really did feel for her, though for myself, I was glad he was off the show and it was pretty clear making an opening for the more interesting if bizarrely unnatural relationship with Spike. Though I care for the characters, I care more for my own enjoyment in front of the screen (shallow, but hey, that's what I watch TV for, entertainment)
I think they could have worked through the vamp whore issue, I think the power issue would have eventually tore them apart, but all of a sudden he was "outta there". That wasn't fair to Buffy, she needed time to process what was taking place, and honestly, I think Riley was looking beyond that, knew they weren't going to end up married with kids, and decided to just get over her sooner rather than later.
She MADE the effort to stop him, but he could have chosen to stay and work it out had he been willing to meet HER halfway. He didn't. I mean, seriously, if he hadn't gone on this mission, no biggie. There will always be another mission and more bad guys to fight. Riley wasn't forced to go, he chose to go.