The problem is "100% defensive" tanking no longer works thanks to the recent Intimidate changes in Update 9; Intim effect only lasts 6 secs and is on a 15-sec cooldown timer. In order to hold onto aggro, you have to generate threat; and while there's threat-boosting gear & abilities, it's still based on your DPS. So no DPS means no threat means you'll keep losing aggro - and if you want to be a tank, it's kinda your job to hold onto aggro.
![Smile](/forums/images/smilies/smile.png)
Plus as Ralmeth says, you won't need or want to be in purely defensive mode all the time. Do you need the DPS of a pure barb? No, of course not; but you do still need to be able to make a significant DPS contribution, IMHO.
My view is a well-rounded tank should have a solid mix of DPS, HPs, Intim, AC, and threat boosts (from class and/or race abilities); if you plan to go S&B, I consider the revised shield feats almost a must-have. [If you find a way to squeeze in any self-sufficiency or extra survivability from high saves, Evasion, etc. so much the better.] Have a look at the Sentinel Squire in my sig. I haven't updated it since before U9, but it gives you some idea of what I think goes into a good tank build and the tradeoffs involved.