As for the heroes:

Junkrat is one of my favorites, especially when there's an entrenched Bastion or Torb's turret is giving the team a headache. Bunker buster and nearest Joker-esque character you'll find in the game. (Hell, a couple of his skins are even Jester-themed). Think TF2 Demo-man, but Australian and totally bat-shit insane.

Roadhog's another Aussie, but as a tank I'd say he's one of the hardest to master, after Zarya (IMHO). Sure he can take a lot of damage, has a self-heal, and can isolate and eliminate members of the enemy team. But one-on-one against DPS-type that knows to keep their distance he'll lose. He's a gank-tank, not a front-liner like Reinhardt or D.Va.

Zenyatta's my favorite healer of the Support set; takes some getting used too, but if you need a focused pocket healer, and a capable secondary damage dealer, pick Zen. His Harmony orb heals incredibly fast, but in my mind he shines with his Discord orb (debuff that increases the damage that Zen does to whoever has it), which can help him annihilate tanks.

Soldier:76 is a good mix of offensive power, mobility, and support healing. I'd recommend him for starters that are coming to Overwatch from other class-based shooters. He may not be super fast like Tracer or have to vertical-mobility of Pharah, but but he can be very effective in the right hands.

Mei is - by far - my favorite character in the game. She may have gotten a nerf in the damage department just after launch, but I still love her. She's cute as all get-out (I recognize that I have a severe nerd-girl fetish and I don't give a damn), and such an annoyance to the enemy team I don't care when I die, because I can usually detain two or three of them, forcing them to deal with me instead of pushing the objectives. Sure, some would call me pure evil for utilizing her in matches (lost count of the number of times someone started cussing me out in the match chat), but there's no other character I'd rather use during those clutch moments when we HAVE to stop the opposing team.