They certainly could have fleshed out the white walker lore more and given us one more season. I think the series needed that to properly tie things up. What we got for dialogue and character interactions was good and quality, there simply wasn't enough time to flesh things out. Oh, and the whole staged "let's split forces, a few of us go to Castle Pyke to check a few things out and... oh, what's that, a boat that's shooting and... oh no! They killed a dragon on their first shot ever shooting a ballista into the sky at a moving target!" Yeah, that part was quite a contrived stretch.