They had no idea how to end anything. They'd show a vision of the future that looked like a really cool series ending then the final episode would be very flat.
What did it for me is they set up a huge Peter - Sylar battle, exactly what people wanted as a finale, then they literally pulled the camera away and shut the door. Nothing of it was shown.
I agree with this thread, the writing quality faded until no one cared if it was cancelled.







