It's not one issue that's hurting BM (IMO, I know many love the changes), but several.
The effect of the heavy-handed GCD changes kill the flavor of the spec for me. BM has been a fast paced run-n-gun style where you set up burst windows and then go big red kitty bang bang for 15 seconds. It feels really good and rewarding when you set it up right. The GCD now severely limits that, and popping CD's doesn't "feel" like it used to.
Without that good bursty feeling, and the addition of micro-managing a tedious, boring, unrewarding frenzy buff makes the spec feel like a lot of work without the "moment of glory" anymore.
The fun just isn't there, but it could be with some minor changes. Here's what I thought the spec should have been for BFA:
1.) Pets: eliminate pet focus, place their abilities on CD's. Pet focus adds nothing to the game. Instead of pet families, have leatherworkers craft collars that go in a pet inventory slot that gives the ability you want. That gives the player the freedom to have the pet they want paired with the ability they want. More choices = more fun.
2.) Cobra shot resetting KC during BW. THIS is the ability that should have gone baseline, not dire frenzy. It's fun, bursty, and feels good. Get rid of barbed shot, bring back dire beast spam, and make the KC reset baseline.
3.) Rollback this GCD nonsense. This does not make the game better. If burst macros are a problem, come up with a solution that doesn't hurt the entire game. On-use trinks were given a small shared CD so they couldn't be used together, and the change didn't rape the entire game.
Just a few changes that for me would have made the spec more fun. If BM needed a bit more complexity, add some dots for us to maintain that don't require staring at a weakaura nonstop for 3 tiers of content.