The basis of my judgement of roleplays has always been founded in depth, the better roleplays almost always carry quality beyond just good writing styles and fancy words. Romance is just one possible dimension to add to roleplay however I've often found that more often than not it's not implemented properly. Beyond the obvious results of pure smut writing and sexual based intentions, it often lacks a realism that translates to a proper perceptible depth. Whilst realism does seem like an odd word to be throwing around in roleplaying, especially with the strong presence of fantasy and science fiction premises, I've always believed a degree of realism should exist. 

With that being said it can also depend on the roleplay itself, in some situations it just isn't necessary nor appropriate but in some it can really add to the depth and therefore quality of that roleplay. My beliefs is that it has to be used properly to be effective, else it often proves more damaging than constructive to a roleplay, a common root cause being a lack of realism. Emotions need time to blossom and grow, thus romance sections can't be written as a spur of the moment thing which is what often happens in roleplays. I think a basic guide for romance in a roleplay would be based in both duration and setting, most likely the longer the characters have been interacting for the more likely they are to foster a romance and also the situational pressures can completely upset the table.

If I had to sum my thoughts in a line it’d be that romance can be a very good component of a roleplay but only if you know how and when to use it.