A good tank is something that is noticeable.

Proper use of your cooldowns and defensive abilities (stoneskin) will allow you to either die a much lower percentage of time than worse tanks, or at least allow you to always have a "cushion" of hp. A tank that never goes under 25% hp allow healers not to have mini heart attacks every 5 seconds. It also allow healers to make more efficient use of their mana -- emergency healing is costly.

Proper use of stance switch will increase noticeably your dps, assuming you spend half your time offtanking due to tank swaps.

Proper use of your other tools will make stuff easier on your whole party (although this is harder to see). Stoneskining the MT, keeping rage of halone up, stoneskining low hp people before a big aoe move (this one has visibility), all this helps. When needed, proper use of silence and stuns is also very noticeable.

By improving on those 3 axes -- damage reduction, dps, and utility -- you can make a MASSIVE difference in a party, one that is noticeable. Of course it will never be as visible than a healer behaviour. But i would argue it is much easier to differenciate a good tank from an average than a good dps from an average (everyone notice the difference between a good player and a bad player anyway, whatever the role).