Gwen can be a hard character to like, but it's not impossible - you just need to understand that she's not in any way intended to be perfect. I think of Gwen as the mary sue we'd all write if we were being honest with ourselves - she's sometimes irritating, nosy, prone to lying, can't let anything go, can't work her way out of a sticky situation, doesn't know how to cook, has a gigantic gap between her teeth and a shaggy haircut and a relatively big butt and no fashion sense, is headstrong and occasionally has poor impulse control, decides to hop into bed with Owen when Jack is RIGHT THERE DAMMIT (ignoring Rhys here because I don't like him), says SHIT when big scary monsters are coming and pukes at the sight of dead bodies. She also, though, has a lot of Velma about her; she has to know the answer to every thing and is good at figuring things out. And of course, as has been hammered into our heads from the first episode, she really, honestly cares about people and is willing to do about anything to protect them. Gwen isn't a vital part of the team because she's so normal, like they keep pushing at us - she's a vital part of the team because without her, they'd be holed up under a water tower dinking alien iPods while the world was ending instead of out doing something useful for the human race.