Clark/Lana kind of were the worst case scenario of couples, no chemistry, no good acting and exactly zero actual storyline. Personally, and probably unpopularly, good writing is the thing I care most about. If a couple has good writing and no chemistry at least I'll still accept them and ignore them, but not be offended by them. Now if the storyline is crap even just on paper (again "OMG, Taylor saw Ridge hugging Brooke and now she thinks he doesn't love her anymore." rinse and repeat 565 times) then I just want to stab it, every time they come on screen, even if the actors are trying their best (Grissom/Sara from CSI comes to mind).

I think too many shows (especially procedurals) treat romance as a preppy little sparkle on top. Something they throw in when they feel like it, not something they craft with care and actually write as an intentional storyline. Something that so often effectively ruins the couple and the characterisation of the characters involved.