I always try to look at someone as a personality. I work with a woman who is very smart, but she never has much besides negative criticism of anyone else or of any work-product of anyone else. I call her and her ilk "Eeyores". Always looking at their comparatively rich lot in life and seeing not what they have, but what they don't have. Or seeing the work-product not in its entirety and what needs improvement, but seeing instead only its flaws and therefore not providing suggestions for improvement, or even the admission they don't have a suggestion, but they have a concern.
Eeyores, my friends. They do have something to contribute but since they look at the world through the lens of their fecal-myopathy, everything looks like crap. I find them mostly very self-absorbed and they aren't really that pleasant to work with. Given the choice between two equally well qualified persons, I will take the positive person. But, given the choice between an Eeyore who knows their technical area well and someone nice who isn't as knowledgeable, I as a project leader, will take the Eeyore and work hard to overcome the crap-vision.
There's a really good article about bad apples at this link:
http://www.freireproject.org/blogs/it-turns-out-one-bad-apple-can-spoil-bunch 
