There are a number of problems with player-NPC relationships in videogames, the most common of which appears to be that they're tracked using a single statistic. In every game I can think of - most obviously Alpha Protocol, as I was just playing it - if you're capable of affecting an NPC's feelings towards you it's a sliding scale from hate (or disinterest) to love, and the actions you take will always slide it one way or the other.
The nice thing about Alpha Protocol is that it's not as obvious when you make a decision what effect it'll have (at least, not the first time through); while taking the flirtatious, suave approach in your early conversations with Scarlet will increase her relationship score, in others a professional option will also help, or the suave dialogue will shave a point off.
While it's still a pretty obvious points scale, it at least moves away from Mass Effect's "choose all Paragon options to unlock love" approach and tries to make the romantic subplots seem like they're happening between real people rather than Thorton and a selection of sex vending machines.
To properly model it, though, you'd need a lot more axes for an NPC's opinion to be measured on. An obvious one could be how your character looks - maybe Kaiden really likes blue eyes and bangs. That wouldn't necessarily guarantee love of course, but it might work similar to a modifier, where it magnifies or reduces the effectiveness of your words and deeds, depending how attractive (or not) they find your character. Likewise, extreme actions - like shooting an innocent bystander or surrendering enemy - could be a total turn-off or major plus if your would-be partner has a particular moral alignment. How you talk to them, and how often, would also be important, and if your actions match up to your stated intent.
Basically, I guess, I want to see a game where doing the right thing for the mission isn't necessarily the same as doing the right thing to pursue a relationship with another character. Sex shouldn't be the end goal, either - depending on the nature of your relationship and your actions throughout the game, maybe you'd hook up with someone and then they'd decide it's better not to pursue a relationship. Mass Effect's relationships kind of had a larger arc than "achievement unlocked: sex scene" as they progressed across the series, but in each game the sex scene signalled the completion of that sidequest.
In the next generation of consoles, there's going to be a relatively minor increase in graphical power, but I'd rather stick with the graphics we've got and see more resources going into building better characters and relationships.