Would that leave much incentive to level it?

You have as much incentive to level this up as you have to rank up any other crit in the game as well as lifesteal.

Are you saying that this just a really strong feature of the hero, or that it is OP?

Overpowered with his carry power allowing him to instantly kill heroes every 10 seconds with little risk behind enemy lines. Even the some of the tankiest heroes in the game need items to survive his burst if he has no items. Add in effective damage items like armlets and bashers he is racking the damage up quickly. But it is a good feature if balanced correctly but it just seems a little too potent currently. Imagine getting walrus punched 3 times instantly followed by 2 seconds of auto attacks with a hero packing slightly over 2 Daedalus worth of crit and surviving (This is achievable at level 8 remember this). You can not out man fight this guy because his lifesteal and damage is too high.

Again, I thought the cost necessitated building mana items, which in turn balanced out their potency. If that is bad design, alright, lets lower the costs and balance accordingly.

Yes that is good balance but being unable to use your abilities is not. If you want to have a high mana cost like it is increase the int will balance this out. If your hero has 13 int it can almost double it with a staff of wizardry. Where if this hero has a much higher int say 20 a staff of wizardry would only increase is only a 50%. So at 9 he has 25 int meaning a rod of atos can double his mana pool but if he had instead say 40 a atos would only provide 62.5% increase to his mana pool. Just because a hero has a high int does not mean it is immune from mana problems.