Fabian was probably the first writer whom I actually saw in the credit box as a young comic reader. Yeah, I don't understand how that works either. And say what you will, as a writer, he earned his chops in the comicbook world, and he played it out very far; he made the New Warriors from scratch, which begat his long affiliation as writing half the X-Comics at Marvel after Claremont and the Image guys took off. But it was his writing elsewhere that drew a lot of attention from me as a kid. I remember reading some piece of Marvel promotional propaganda where he wrote his "Two issues rule"; If you aren't being entertained by a comic series for two issues in a row, stop buying it. I think I was twelve when I read that and it stunned me: a MARVEL writer is telling me there's a time when I should get off comics? Really? Wow! Ever since I've applied that rule pretty rigidly. This probably goes a long way to explaining why I don't buy any more from Marvel: I mean, when was the last time that any given two sequential issues even started a story anymore?
In recent years, Fabian's output has been hit and miss: His Thunderbolts was usually passable, but it excelled when it focused on the characters you knew he had developed some complex characterization for, sometimes even exceeding the development started by Kurt Busiek. For me, the character work he did with Hawkeye, Zemo, Songbird, and Atlas still really stand out for me. Each of those characters in his work had a singular and unique voice to me, and each was a part in what I felt was the larger story Nicieza was trying to tell about the nature of power and heroism, and how nature and nurture effected the path you took.
Fabian's strongest skill, I think is that he respects and tries to bring out the best in the characters he's given. No character, no matter how obscure, would be used as cannon fodder for his stories. In fact, he's shown a remarkable gift for writing a compelling narrative with just about any assemblage of random bottom-tier characters (see his New Warriors run, and then his Thunderbolts run, especially the end there where he was using frikkin' Plantman and the Gypsy Moth. Seriously, the Gypsy Moth?). DC could probably give him a team consisting of Gypsy, Flying Fox, Wild Dog, B'wana Beast, Nightblade, and Lady Quark and turn it into a crackerjack series. In fact, I think DC should offer him that series right now.