Yeah it's another case of writers exaggerating a particular part of a character and making it practically their whole personality (or in your Not Going Out example it feels to me that they've spread Toby and Anna's relationship to the other couples).
I think I've used this example on the BCG forums before but it's like Denise and Dave in The Royle Family. Not the brightest of sparks to begun with but after the initial run of 3 series and the two Christmas specials, it was like Aherne and Cash decided to lobotomise them. Before, though not intelligent, they relatively had some common sense and quick wit to them.
I still enjoy Two Doors Down and feel part of it is potentially intentional and hopefully character development what they did with Cathy and Colin in the finale and -SPOILERS- them being brought down a peg or two.