FYI, I'm basing the following numbers on two of my characters. One is a Clr6/Radiant Servant8/Contemplative3 of Pelor with the standard 4 domains (17th-level total). The other is a Bbn1/Ftr6/Ex-Mnk2/Dervish6 (15th-level total).
Attacks: The cleric can get to the mid 40s to hit, with 5 attacks a round, without any help. The dervish can get to the mid 30s with some buffing help.
Damage: The cleric does about 30 points a hit without any short-term buffs. With them and power attack, he can do about 60-70 points a hit. The last combat he was in, he did about 300 points in 1 round against an advanced runic guardian. The dervish does about 30 points without any power attack, and up to ~60 with it. He also has a much better critical range (15+), so that factors into play a lot.
Saves: The cleric's lowest save is Reflex, at +15 IIRC. His Will is +31. His Fort is in the mid-20s, IIRC. That's before short-term buffs. The dervish's low is Will at +12, and his high is Fort at +19. Again, before any buffs. With buffs, his Will is in the mid-teens and his Fort and Ref are in the low- to mid-twenties.
AC: Maybe I'm just good at coming up with high ACs, but the lowest either of these guys is ever at is 31. The dervish's walking around AC is usually 41, and he can crank it up to about 60 by sacrificing offense. The cleric can get to the mid-60s or maybe 70, although he doesn't need to sacrifice offense (he doesn't use Expertise - he just has tons of armor, natural armor, etc. thanks to spells - I'm not actually sure how high he could get it these days... maybe I'll go figure that out).
Resistances: The dervish doesn't really have many other than Evasion and Close-Quarters Fighting, unless he's received buffs from his friends. The cleric is immune to just about everything most of the time (he's powerful to the point where he isn't all that fun to play anymore, so I'll be retiring him to NPC status at the end of the year).
Now, with these things in mind, here's where I see it: at 16th level, tanks should have standing ACs in the mid-30s. With help from their friends, they should be in the mid-40s* at least. Their attack bonuses should be about 30. Their lowest save should be at least a double digit. They should have some way of being resistant to energy or at least avoiding area-effect spells. I've heard a lot of people say that AC is irrelevant, since at high levels you're going to get hit regardless. That might be true of your opponent's first attack, but if you can avoid those iterative attacks, that's good.
(* mid-40s isn't all that hard to do at 16th: 10 base + 13 armor [+5 full plate] + 7 shield [+5 heavy shield] + 4 deflection [ring of prot or shield of faith] + 5 natural enh [barkskin] + 1 Dex gives AC 40, and that doesn't even include polymorph/alter self or a dozen other ways to get AC; the +5 armor and shield, btw, are from Magic Vestment + Divine Spell Power)
Anyways, just some random thoughts.