Regarding tanks, a bit more then 200~250 resistances (MR and Armor) and around 3k health is usualy the way to go, in general. When I go tanky dps with jarvan (if I go as far as finishing the build) I end up with over 220 armor and mr and nearly 3,5k health. As a full tank and champions that benefit from having resistances, you'll probably want more resistances even, giving priority to one type over another depending on the enemy team. You do need health, though, but you'll rarely go over 3,5k health, resistances are more important, overall, but health is more useful early in the game and you'll need it also, but that doesn't mean you should get a warmogs on every tank ever, like some people do.
I think the max hp you should go for as a tank is 4k hp, only mundo can go higher and not seem silly, and only cho'gath can hit that value without it (I think). Thats the extreme end of the spectrum though, 3,5k is probably the ideal value.
NINJA'D:
I find the use the term "diminishing returns" to be really unfortunate when describing armor and mr effects. There are diminishing returns regarding the percentage shown, but there aren't diminishing returns regarding the overall damage required to kill someone with X hp + X resistances. There's a huge argument going around the riot forums over it and both sides fail massively to explain themselves.
So yea, if you have 1000 hp + 100 armor, it increases your effective health by 100% against phisical damage, meaning it takes 2000 phisical damage to kill you. In this sense, there's no diminishing returns. But the % of reduction shown when you check it in game does show diminishing returns.