Let me give you some evidence, then:
One popular thing is the "command counter" thing (from adepticon, I think?). Give a squad the counter, that gives them X power (FNP, scoring, various other little goodies). If you have a Tau army with nothing bigger than a FW squad, that's pretty crappy. But if you're playing Draigo-wing, it's stupid awesome.
Low Gravity? Well, that could mean a few different things, but sometimes it means extra movement for free. That's usually pretty awesome if you're an assaulting army, and awful if you're a shooting army.
I remember planet strike had a stratagem that made these little walls that hindered movement. You could remove them, or maybe it was just move through as normal, with grenades. The problem with that? THE ENITRE IMPERIUM HAS GRENADES. Only a few Xenos units even have the option to get grenades, and they usually have to pay for them. Seriously, like the humans super power in 40k is bringing grenades.
"Fluffy and Interesting missions" will only lead to "thematic list building" if people know them ahead of time. Revealed day of, you're really just helping to pick winners and losers by random chance, rather than skill. I'm alright with revealing missions day of, but then they really HAVE to be as vanilla as possible.