The difference in gameplay going from 6v6 to 5v5 is the loss of off-tanking and the more complex gameplay that used to be possible with tank ability rotations, tank combos, peeling, taking alternative side routes on the map, and one (or more!) tanks using the other tank as bait for springing an attack or a tank+DPS ult combo. Games weren’t reduced to boring straight rock paper scissors mechanics for tank players, and games weren’t so affected by individual bad players, throwers, DC’s, or simply tank diff.
5v5 games play more like deathmatch. Fights are faster as it is easier to be picked off. A fight is a loss for the team whose tank dies first in a fight.
The off-tanks from Overwatch 1 all feel and play like they were shoehorned into 5v5, given their kits were designed with playing in cooperation around a buddy tank’s barrier and abilities, and given they offer little to no team protection, save for some momentary burst protection abilities of limited value to most teammates, in comparison to proper barrier shields. Doomfist was a real head scratcher for me to understand the logic behind his rework. Here was a DPS changed into an off-tank, when the off-tank playstyle was being deleted and off-tanks lazily buffed to avoid being totally obsoleted by barrier shield tanks.