King cobras are almost universally recognized among snake enthusiasts as the most intelligent of serpents...I had a friend who's king helped him study for a chemistry test, and guess what, he got an A!
Seriously though, their intelligence can be either beneficial to the keeper or downright dangerous. Kings become rather accustomed to regular husbandry routines fairly quickly, and learn to anticipate a keepers actions when around their cages. They can be considered to be "calculating", which is not a trait often used to describe snakes. Some peoples kings behave rather well, and don't cause too much of a problem. However, as B.W. mentioned (and he's not the first person I've heard this from), they sometimes single out a handler and act completely different with them than they do other handlers...and it really sucks for you if they don't like you, because a 12-15 ft. venomous snake who actually WANTS to kill you is not good.
As for the venom, as mentioned before, it's not any more toxic (and might actually be slightly less toxic) than true cobras, the sheer amount of venom they are capable of injecting in a bite is TERRIFYING. Death can result within 30 minutes from a large dose, and kings are not really prone to delivering dry bites. They also like to bite and chew, or do a "walking bite". Of course, this means massive doses of venom being injected and if IMMEDIATE medical treatment is not sought, death is almost a guarentee.
That being said, kings are one of the most magnificent animals to walk, er, slither this planet, and it's tragic that their populations have been depleted so much...due to habitat destruction mainly.
I plan on keeping and hopefully breeding O. hannah one day, but I think that's a few years away...