Warbringer
Explorer
I really don't want stacking of advantage/disadvantage. The way they do it now, where stacking just doesn't work is a good design in my opinion. Stacking is one of those things that just slows down combat* and in certain situations make rolling the dice superfluous, something I dislike.
The slowdowns come when you have players going: I got advantage from x... starts rolling, stops, add advantage from y, wonders if he has advantage from z and after rolling begin discussing if he should have had advantage from æ as well. 3e was horrible in this regard with often up to 5-6-7 different sources of +1 and +2 bonuses.
I would rather see maneuvers and such that allows you to do new things instead of adding a small bonus or another advantage dice.
So, we've been playing multiple d20s for about 4 years now (basically an exchange of actions for a d20) and it doesn't slow down combat in the slightest. W
We also play "successes" so if you hit multiple times with the d20s, or by more that 5' you get benefits from a great hit (FYI this is our critical system). These benefits can be used for more damage, maneuvers, or even aid a companion. Stunts are simply a target number of successes now, eg " I want to swing of the balcony, land on the ogres back and backstab him"... No problem, you need 3 successes...
Speed of play is not a factor.