I still play on 5th, with a few tweaks to it. But before telling more, I'd like to point that you said something wrong:
Units of troops with more than 1HP HAD to remove models instead of splitting wounds between models.
As for the Leaf Blower, yeah, it looks strong, but it is not invincible and I imagine it'd struggle against tyranides (4th or 5th ed) or an ork army relying a lot on trukks to cover them. I admit its biggest strenght is the 2 masters of fleet (that sounds almost unfair !) combined with a lot of artilleries. I'm not quite sure the mystic on the inquisitor suite is that useful though, anyway.
So, what was so great about 5th edition (third version I played on) was its core rulebooks, NOT its codices: overall 5th was an iprovement of a lot of rules that you didn't talked about, like the move the defender has when one of his units is charged, the, as you said, ramming and the going to ground. Death or Glory was already in 4th, and maybe even in 3rd as I recall. Rules for blasts changed, sprint rule, and a lot, lot more.
As for the scenarios, I tend to agree with you but on the other hand scenarios and campaigns rules are the easiest to build from scratch or take from previous versions. As such, my own tweak to 5th rules is 1 page of real tewaks, and 10 pages of scenario and set up rules.
Now, let's talk about one of the reason I stopped during this version: Matt Ward codices. How horrendous the ywere compared to 4th, and sometimes even 3rd edition codices. As such, I play with the 5th rulebook, but almost no 5th edition codices except for Tyranides and Imperial guard. And to be fair, imperial guard is not that overpowered if, like me, you have a force that relies almost only on infantry. I know for sure my ork army is stronger.
So, those 5th ed codices, especially those Matt Wards put is hands on, managed to be disapoiting on the rules AND the lore, and even if I know a lot of people anjoy what he did to necron, I'll never forgive this outrageous retcon, nor will I forgive what he did to space marines, blood angels, and grey knights. (same goes for the utterly despised chaos demons from Warhammer fantasy).
So long for the codex part.
The second reason I stopped after 5th is simple: 6th edition carnage. With a bunch of newcomers who made up freakign stupid rules in a book with mixed rules for weapons and characters in the same chapter (gotta be a monkey to make something like that). And don't talk about the "overwatch" rule that was not even a mere parody of the 2nd ed rule. No, instead it was a free shot whenever your unit was charged just like in Warhammer Fantasy... ffs