I dunno, I see the complaints, and I agree that overall the season, and the finale have been pretty weak, but I'm more than willing to write it off as mediocre and hope for the best for next year.
Re: Dean / Lisa / Ben - Haven't Sam and Dean ALWAYS been short-sighted about these sort of things? (Dean taking a deal with the Demon to save Sam, Sam working with Ruby and taking the Demon Blood, Sam going off on his own because he feels guilty about starting the apocalypse, plus all the episodes that show how stupid they can be in regards to loved ones). Dean thinking that erasing their memories would be the best plan of action isn't smart, but it certainly isn't *character assassination* (sidenote: reading around the internet this morning, that phrase is waaaaay overused in regards to showrunners and plots and favorite characters - just because you don't like what character ends up doing doesn't make it character assassination. Matt and Trey dealing with Chef in South Park? THAT's character assassination). In the grand scheme of things, it felt like they were just trying to figure out a way to get rid of Lisa and Ben (an admission that that plot strand didn't work), and do it cleanly. Worked for me...I seriously doubt we'll ever hear anything from them ever again, and I'm cool with that.
The only reason the Cas thread felt rushed or unearned is because the majority of the work was done offscreen. The big Cas episode 2 weeks ago did a lot of good to set this up, but, probably should have come earlier. I would have loved to see more angel/demon battles and monsters, but as I said before, the budget nowadays seems to be nonexistent. Next year they'll be filming on Flip Cams and using sock puppets for monster designs. What we got, worked, IMO, and like I said above I hope they can go forward with it and turn it in to something worthwhile in season 7.
Sam's wall coming down was pretty weak, in that the effects weren't as catastrophic as we were led to believe. But, outside of the buildup, that plot thread was pretty weak as well, and I'd rather they dealt with it and moved on in this matter than waste time on the after effects for 12 episodes in season 7. Like Lisa and Ben, it shouldn't be a major part going forward, despite the importance placed on it earlier in the year. If they wanted it to be more important, they should have devoted a whole episode to it, a few episodes ago, and not in the finale, for sure, but it wasn't the worst thing ever. It was a solid idea, with average execution. Which kind of sums up the season. I get the sense that they were kind of rudderless and the beginning, and threw a whole bunch of different ideas at the wall to see what would stick. Once they knew that the fall of Cas was going to be the big story, they needed to break off the other plot strands as quickly and as easily possible so that (HOPEFULLY) they can move forward in S7 with a clear direction and a strong finish. The whole second half of the season felt like course correction, and the finale even more so.
Considering how aimless they were for the first half of the season, I think the finale, despite not being that great, did a lot to make me think that the showrunners understood what didn't work and what did, and can right the ship next year.