There is some slight overlap with some characters but the differences between classes are noticeable when you maximize their abilities. Bastion is definitely a defender role and can out damage a Torbjorn. Before the open beta,Torb and Bastion were very similar and a common complaint on the subreddit at that time. I find Torbjorn more of support like Symmetra since she also provides armor and creates turrets.
Dongles had to look this one up because we didn't know what the number under her crosshair was but Zarya is very strong when her energy is maxed out. It is not explained in the game but when use your shield, it absorbs that damage the shield blocks and charges her up. It maxes out at 100 and signifies how much damage amplification you get, resulting in more damage output. She probably does the most damage out of the tanks. Roadhog is more of ability/hero blocker. You can grab bastions in turret form, pull Pharah's from the sky, grab Junkrat tires to absorb the damage away from the team, etc.
McCree is my favorite because you get free kills with him. Yeah you can free shoot if you want but you might get one or two kills a game like that. You have to stun them with the flash bomb and right click to get the "fan the hammer" kill. This can take anyone out with one volley unless it's a tank at full health. Even if it's a tank they won't live if you have a teammate there. On that note, I find McCree and Reaper similar because they both need to be close to maximize damage. I think I only played Reaper once in the open beta. Even though there is overlap, that is what Blizzard is good at doing... balancing and making changes. Over time there will probably be several changes to abilities, especially when it gets competitive.
This mode also rotates every week as well. The the stress test before this I think there was faster health regen with faster ultimate regen and damage amplification.