I don't think it is completely story fault. It was a great show, which hooked people even after so many months of disastrous story line.
What ever excuses we can give for Sarah, we can give the same to the role of IP. His role also started same way as Sarah's role. Bad guy Don, wife beating person, whom his own wife tried to kill. Who might be the killer of lead Prithvi. But this role got sympathy of most of the audience. Of course not all.. :) Still every body appreciates Adi in his negative role. When he fell in love for Meera, everybody thought that it is pure love, and it is not his fault to fell in love with Meera for her pure love.
But Sara is a compelte failure. Even when she cried or scared, nobody is sympathetic. When she fell in love for Prithvi, she couldn't portray as true love. Her appearance, walking style, dialogue delivery gave the feel of an item girl, but not a woman who needs second chance in her life. she neither liked by people as other villains like kalki,lakhan, nor had some sympathy ( to some extent) as Akash. I completely take it as her failure in that role. If she is successful, eventhough we want MITHVI together, we should feel sorry for Sarah, for not getting Prithvi.
agree with u she din portray the character well.