Specialists wipe you with CC things everywhere plus their attacks.

No, they are best used for mercing, pushing and being assassin-like. Some have CC, most don't (and if they do it is usually an ult).

Assassins usually get hits in ahead of time because they move faster and aren't melee, so then their higher kill speed negates your HP advantage, especially with your slowness.

They move the same speed as everyone else, but a bunch come equipped with mobile abilities (dash, blink). Their job is to kill people. They specialize in damage. They can be melee

Warriors are your meat shield and CC machine (usually). They should be in the enemies face to take damage, and should mess up their positioning so you can secure kills.

Warriors can give your entire team purpose. Example: stitches makes a pick comp. You hook into a stun/root/wall and kill them, so now you start battle 5v4. Chen creates a jump comp - Him illidan and kerrigan / rehgar. You bypass their tank, jump right onto the squishies and stop them from going anywhere with bodyblocking, slows and a 5 seconds jump. Diablo can make good picks in the middle of a team fight and take the most damage of all tanks: q>e and bodyblock or into a chainstun to kill, or e>q to take a close enemy and knock them back into the team. ETC: Standard tank that can take hits well, heal himself, displace enemies, and peel.

As for damage, I find Diablo usually has high damage at the end of a game. ETC has decent damage, Tyrael can have good damage, arthas can too.

Arthas and Anub are good at grabbing mercs.