I think D&D really were on to some solid ideas for how to improve the story, they just failed to execute on them.
Cutting Aegon to speed up Tyrion is a great move, and I've liked Tyrion more than his book counterpart this season..
Sansa in Winterfell could have been good. Never cared much for the Vale plot, but had they adapted its main thrust of her learning to play the game and gaining allies among the lords, it would've been great. Sure, there would be unpleasant scenes with Ramsay, but it would've been tolerable because we could see her executing on a plan to take the Boltons down, instead of just waiting miserably for Stannis. And Theon should've provided a great opportunity to showcase her fundamental kindness, even in terrible situations, as she helps along his reawakening so he can help her in the end. Instead we got no Northern lords, no action from Sansa, rape, and Theon has regressed even further and committed even more heinous crimes
Jaime in Dorne could have worked just fine if they wrote Ellaria and the Sand Snakes as something other than villanous cartoons. Cut the stupid bucket-whipping, merchant-killing scene and instead just have some actual character developing with them talking about why they cared about Oberyn so much, and coming up with a smarter plan than "murder Myrcella!"
I like Cersei's plot so far, but it could have used some set up for the commoners' increasing religiosity leading up to the Sparrows last season.
People would have hated Meereen no matter what they did.
So yeah, I think they actually hit really close to a general framework for a condensed season that would have worked better than the books. They've convinced me that's totally possible, it's just not what this season is. Which is worse, because I can't just say "Well, the book was unadaptable!" They just missed the mark and the result, so far, is bad.