The bad writing, IMO, is how Jon Snow went (that's like the president flying an F-18 in Independence Day), how they just to happened to be saved by that frozen lake, how the timing was just right so that the dragon came to save the day at the last moment, how Benjen showed up suddenly, etc.clinker03 said:
The idea around the plan was fine but given the context of what they were trying to accomplish I thought it was foolish. I mean it sounds great - "Let's just go grab one of these things and we can show it to Cersei and she will have no choice but to join us - yay!" It's great in theory but in this show's reality its not that easy. And when they were planning it there wasn't even a hint of a counterargument - everyone was like "sure, this sounds great - let's do it". I felt like the show writers needed to get a dragon in the hands of the Night King and this was the easiest way from them to do that. It felt like lazy writing. Or it could be they were hard pressed for time given they only had a limited amount of episodes to work with so they were speeding things up.
I disagree on the idea of the mission itself being a bad idea. Cersei would never agree to join forces on their word alone. The only shot they had was to take her north of the wall (which would never happen) or take a wight to her (and even that didn't convince her enough). So I think trying to kidnap one was a good idea, not a bad one.
What should have happened, IMO, is that they send 5 or so dudes to covertly trap a wight and sneak him back, rather than go to war with 100,000 of them. Of course, that doesn't kill a dragon so the writers would need a smarter way to get that figured out.