He's a jack-of-all-trades kind of guy.

Never masters a damn thing, because having a bunch of probably useless skills, is more interesting for him than mastering one thing. It's not even that he grows bored easily, but his chronic insomnia meant he had a lot of time to explore. Hobbies, skills, trades, etc. He hates all of them but he knows the basics.

He's put that damn ship in a bottle, watched baseball (he got bored, fell asleep, and spilled his expensive beer), and taken every appliance apart that he owns. Which is, admittedly, not that many. There is one thing, he is considerably better at than others, and if it were a skill marketable anywhere other than NCIS, he'd be a master.

Never being who you think he is.

He has that in spades.