I also played as the new heroes a bunch at Blizzcon.
D.Va - Seems pretty strong, tends to fill in gaps the other tanks have, but also has weaknesses they don't.
Almost her entire health pool is armor - makes her much better against SMGs and especially shotguns, so she deals with a Reaper way better than any of the other tanks.
Very hard to finish off - She pops out every time her mech dies, and can often squirm away if she had decent enough positioning. Gets her ult back up really quickly and she's good to go again.
Very mobile for a tank - Her boosters on a 6s cooldown, she can go all over the place, very quickly.
Her ult is ludicrous - unbelievably large AoE, seems to one-shot basically everything. Takes awhile to blow up, so good players should mostly be able to get out of line-of-sight to survive, but easier said that done. More importantly, seems like it can serve as a strong counter to Mercy and Lucio's ults (which are way too useful currently). Also fantastic synergy with Zarya and Mei's ults.
Very easy to focus down - That armor isn't nearly as useful against many other weapon types, and she's crazy easy to burst down on Pharah, Hanzo, Widowmaker, etc. The other tanks all have barriers to help them survive more consistently, so in practice against most popular team comps, she actually feels much LESS tanky despite the armor.
Her gun feels mediocre - Infinite ammo doesn't make up for the damage potential, and the slow movement just feels too punishing. May be okay considering she's a tank though.
Defense Matrix - Seems really cool, but in practice just feels like a worse, directional Winston barrier.
Genji - I think a very high skill cap hero, you need to have very good movement and decision making to get the most out of him. As such, hard to list definitive pros/cons, as it all feels kind of up to how good you get at playing him.
Movement kit - Incredibly awesome to be able to double-jump and air-slash, in addition to his Hanzo-like wall-climb. His slash comes off cooldown for any kill, not just slash kills, so I see potential for some DBZ-esque fight movement in the right situations.
Deflect - Particularly awesome for dealing with entrenched snipers and soul-crushing for poor Torbjorn (can't make his turret stop shooting). Seems hard to use well in other situations, but obviously still very powerful in the right hands.
Super glass-cannon - Feels like he will need good support like a Mercy, or at least some shields armor from Symmetra/Torb to go ham. He's easier to catch than a Tracer, and not much more durable, making him kind of an easy target if you can keep him at a distance. Skill going to be a major factor here though, his movement kit should help him if used well.
His ult - Despite it's awesome damage, it encourages you to be right in the face of your enemies, which isn't always a good idea with his durability. It still has great potential though, the sword is a pretty big frontal cone aoe of massive damage. You just can't pop it and expect everyone to feed you though, they'll likely just kill you instead.
Mei - My favorite. So fucking fun, great disruptive playstyle. She's going to be super annoying to deal with, and is going to enable a lot of interesting strategies with her wall.
My favorite moment playing at Blizzcon was holding off 6 people on the final control point completely on my own, even killing half of them in the process while waiting for my team to revive. Freezing 6 people with Blizzard is pretty damn satisfying.
Disruptive - There aren't a lot of characters that can just jump into a huge group of enemies and expect the situation to turn out well, outside of maybe Winston with his ult up. Mei can do it though. She can break groups up, delay and distract, and get plenty of kills in the process.
Blizzard - Probably OP honestly, especially on point-capture maps where the enemy team is often forced to stand together. She can't easily be punished when she can wall herself off and/or put up ice block. Synergizes with other ultimates or even regular attacks from teammates better than Zarya's ult imo, as they can't even attack back while frozen.
Her damage sucks - Not a big deal, not really her role. She still has plenty of kill potential with right-click headshots or just dragging fights out with ice block and freezing until she can finish a target off. I got several 20+ kill streak games at Blizzcon with her, with most of the final blows. Against good players though, doubtful she'll be the one you are looking towards to carry you.