I actually really loved the finale. It was a nice change from the season 2 finale where all the characters ended their relationships and parted ways. We got to see some "happy" endings, even if Freddy was dead. I think it's always important to remember that skins is never going to wrap everything up nicely. That's just not how the show is set up. Some things were given closure (Naomi's character, for instance), while others were left open to interpretation.
Any other show would have focused this finale on everyone's reaction to discovering that Freddie was murdered, but skins focused on the rest of the characters lives apart from that. I thought it was refreshing to not have to get into all the aftermath of Freddie's death--obviously everyone will find out about it sometime after the last scene of the episode, and we can all decide what happened for ourselves.
And also, despite the fact that she's Freddie's older sister, there was no way Karen was going to be in the new cast. Like she said, she doesn't share the "skins" interests. She wouldn't fit in.