In my opinion, you can't look at Tanks without comparing to both Brutes and Scrappers. Brutes should fall into the middle between Tanks and Scrappers. If Scrapers are a 10 for damage and Tanks are a 5 for damage, then Brutes should fall in around 7.5 (and should never be able to reach 10). If Tanks are a 10 on survivability and scrappers are a 5, then Brutes should fall in around 7.5 (and should never be able to reach 10). If a Tank gets most of its defensive abilities early and scrappers get some of the best late (30+), the Brutes should get them somewhere in the teens to early 20s. Etc. Rinse, repeat.
I'd much rather see the Tanks have their unique tank abilities improved than generic damage boosts. I would, for example, much rather see a Tank have a higher threshold on taunt-aggro than Brutes or Scrappers. So, for example, a Tank can aura-taunt 10 and punch-taunt an addition 5, maybe with a (small) damage boost based on actual taunted numbers. The Brute should be less than that, maybe 5 for an aura-taunt and 5 more with punch-taunt and they get a (small) survivability boost based on actual taunted numbers. The Scrapper should have the lowest numbers of aura-taunt, say 3-4, and punch-taunt of around 1-2, for no more than 5, and maybe something more Scrapper-ish, like a (small) bonus to power recharge and/or endurance recovery. Now, each of these feels different, holds and uses mobs differently, and plays differently in a group. Excellent!
This allows non-numbers crunchers (like me) to see the value of each... A scrapper is going to provide me with a lot of up-time and damage, but the survivability is much lower (but still better than a ranged or support archetype)-- making me good at solo play but wanting to team for the heals/defenses and crowd control. A tank provides me with a lot of survivability and helpful crowd control when in a group, but my damage is never going to be the best so it will take me longer to wade through things -- making me good at solo content but wanting to team for their damage. A Brute is a happy medium-- I'm a happy medium in damage and crowd control of the melee classes making me good at solo but I want to team for *any* benefit another character provides (more damage, healing/defenses, better crowd control, etc.). Also, people creating teams can really consider what they need for a TF or missions, as if they have a lot of CC or group defense to start, then maybe a brute or scrapper is a better choice but if they have limited or no CC or group defense, then a tank (or two!) is better. Having all three even has upsides for a team!
Right now on live, it feels like you've got two classes that are nearly the same in defensive capabilities and taunting ability (tanks/brutes), with one version doing more damage, or you can pick the other archetype that does the most damage (scrappers) and is slightly lower in survivability. In practical play, there is little difference to survivability and crowd control as I switch between my brutes and tanks, especially as I ease into the 25+ game environment. I only see a difference when I switch from brute/tank to scrapper and back. In my opinion, I should feel a difference (and teammates should too) between a tank, brute, and scrapper and adjust my (their) play accordingly.
Hope this opinion helps the discussion.