I have an answer for that...
You start off at 20k mana, and as you use it, you replenish it, but the fact is, it is better to be at 20k mana than 0 mana.
For a warrior or druid, it is the reverse. You have 0 rage, and have to build it up.
A dk is different than all of them.
It possesses a rage system, probably very much like the one now adopted for warriors and druids, but it also has a rune system. It can be rather confusing to use both of these systems at first, but as you catch on, you realize it is not as difficult to use as rage, because you still have resources ready at the pull.
Dk's rune and RP resource system is different then most other classes system, but it's not if you look at it the way it actually works out.
not many abilities require RP _AND_ runes. So if you know which abilities you have to pool RP for, and which abilities you have to pool runes for (and which runes, and which abilities you can use to regain those runes quickly) it's not really that hard of a system to adapt to.
Mind you I've never had any more of an issue with my DK vs. my paladin but i don't raid tank on my DK. I mostly just leave it to 5 mans. But i still play it the same way, I might only use the DK for 5 man tanking but I'm not one of those scrubs who runs with no gems and enchants ^__^
I think you missed the point. Rage is a resource that warriors and bears have 0 of at the start of a fight, correct? Aside of popped abilities, orleft over from the fight before, it will also decay over time. Mana does not, we can agree on that, correct?
A DK has a rage similar ability. Probably more similar now, than it was. Runic power was generated as we hit things or when we used certain abilities. It also starts out at 0 and decays over time not in combat, the exact same way a bear or a warrior tank does, correct? OK, so, if DK's didn't have runes, the 3 tanks would be incredibly similar, and have to start rage regeneration at the pull, but DK's have 6 runes, also. These refresh, in that sense, similar to mana, but entirely different. And it is also way different from holy power, too. DK's do not start a fight without any resources the same way a warrior or a bear does, they have 6 runes, while pallies have 20k mana.
It really doesn't matter that no DK ability requires both runes and runic at the same time, the point I made was, runes regenerate, not decay.