Shikamaru is far from being a dedicated ninja, in fact his interests only lie in that area because he wanted adventure to a small degree but at the same time keep his life normal.
But don't let his lack of motivation fool you, he is a talented tactician and talented shadow ninja. Though he may not be as strong or have as much chakra as the other, more powerful fighters, he does have a rather good control of his chakra and he can use everything to his advantage.
However, his skills are only in the shadow ninja realm which means he can only really fight with shadows. He can bend, shrink, and extend his shadow (but he can only expand as far as the shadows normal area would be), which doesn't seem like much but he can use shadows around him so he can freeze enemies in their tracks. Basically, it's the only jutsu he really knows.
He isn't a very good fighter, hence the use of shadows to disable his opponents.
Though, with his obvious defiencies in some of the areas of being a ninja, he has intelligience with an IQ of over 200. No one knows this except Asuma because Asuma tricked him into taking an IQ test (by disguising it in the form of a game) and pretty much everyone doesn't pay any attention to him because his grades were as low as Naruto's! (Yes, that low!) That's the price he paid i suppose. ^_^ He even admits that he's just a side act.
But what is really good at, other than being a leader and knowing what he's doing? Why, it's running away. Since his tatics are basically last measure tatics and delaying tatics he become the expert of getting his ass out of wherever he is that is unpleasant.
On the whole, Shikamaru is an interesting fellow. He thinks heroism is immodest and, for the most part, foolish. Which is true, mainly cause he's the genius at running away. However, when the time came for him to be a hero, he took the job. Mind you, he didn't relish it especially because he didn't want die, but oh well. Thank goodness Asuma saved him! ^_^;; Phew!