... for:
1) Keeping the nasty mobs from hitting me
2) Giving me vigilance, and therefore allowing me to unleash just a touch more kickass
3) Pulling mobs/bosses away from black/purple/whatever colour circles, so I can stand behind and safely dps
4) Confidently leading the run through without displaying any hesitation, thereby making my experience that much more enjoyable
5) Marking the mobs you're going to hit the most, so I know what you're doing ... and
5.1) Keeping that mob as your main target and not jumping around between mobs.
6) Your commanding shout, which helps us withstand aoe damage just a little bit more.
Those are the few things that I missed from a good tank when we had a tank last night that did none of these (multiple wipes in heroic FoS FTL), and I realised just how much I appreciate a good tank when I get one.
If you do these things (and other tanky things, of course), thank you ... if you don't read this thread and I'm sure there'll be more tips forthcoming as to how to make your party member's lives a touch more enjoyable.