Basically, flying stacks. Based on Jetpack.

What other abilities don't stack, but conceivable could? Lifelink did for a short while. First strike + first strike = double strike? This could be a cycle.

Deathtouch also did for a short while, but it doesn't make much difference :P

First strike + first strike feels like it ought to equal firster strike. As in, say there's a combat with a creature with 4 copies of first strike, some with 3, etc. First all those with 4 instances deal damage; SBAs and round of instants; then those with 3 deal damage; etc; 2; etc; 1; etc; and finally those without any at all.

Intimidate might be able to stack, in some way a bit similar to this card. "Intimidate, intimidate" on a red creature means it can't be blocked except by creatures that are both red and artifact? Or red creatures with an additional effect making them red as well? :)

Oh, and of course, we know what multiple instances of haste has to mean :D