Torri Higginson as Dr. Weir when I first started to watch the show did look good. I felt the DR. Weirs charter was going to be a strong female role, but her character started to take a back seat in Season 2 to the others. She when from a starring role to a secondary one.
They also did not improve her character like they did the others.
The show became too much about just a hand full of character, Sheppard, Teyla, and Ronon. Don't get me wrong I like them, but it just became about them too much, but what the show did to Weir they did to others: Dr. Carson Beckett, Lt. Aiden Ford (I think they got rid of him to make room for Ronon).
When a show become too much about few character and the plot is just fighting and shooting and not about scientific discovery or anything else like SG-1 it's going to go downhill because its get boring.
This is why she left the show at the end of season 3.
She just had enough.