Warriors were the best (only) raid tanks in Classic because of one skill: Shield Block. Assuming you had 25% of basic avoidance: parry, dodge, miss, block then Shield Block made warriors uncrittable (200% damage of normal hit) and uncrushable (150% damage of normal hit) by raid bosses.
Paladins could get uncrittable by using a mishmash of gear by stacking defense rating. A feral druid could never become uncrittable. Both paladins and druid could never become uncrushable because of the lack of a Shield Block like ability that would push crushing blows of the bosses attack table (defense rating reduces the chance to be crit, but not the chance to be crushed).
It's exactly these critical and crushing hits (if you get two in a row) that kill tanks in raids. The chance to get a critical hit from a boss is about ~5%, the chance to get a crushing blow is about 15%. This makes the chance to get 2 consecutive crits or crushing blows about ~4/100. This will happen a bunch of times (~5) during a 5min raid boss fight.
My suggestion for Classic would be to remove the critical hits and crushing blows from the boss attack table (and slightly increase their damage to compensate), (or give prot paladins and feral druid a skill to do that). This would remove the problem that is holding back these two tank specs. (There's also the topic of the gear, but changing that would be a no-go in my opinion.).