Not because of his convoluted language, which entices and charms the converted but puts off others (though linguistic innovation is a way to provoke new thinking), nor the ways some of his (and Guattari’s) concepts get taken by their followers into a celebratory Mad Max style of desert anarchism (though desert anarchism sounds okay to me, at times & for a while, just not as a model for social and political life).
But because of his willingness to think, to forge new, usable concepts in a space that’s free of presuppositions about what’s natural and what’s cultural, what can and what can’t be done, and in a way that makes the natural and the cultural, the political and the psychic/spiritual, open, maximally porous, and non-predetermined. Deleuzian thinking urges a fluidity with concepts, with structures and systems, as it creates productive textural mash-ups of the political, the psychic, the spatial, and the bodily and biological.
So while his books with Guattari are the best known, I would start with his work on images, cinema, thought, Bergson, Spinoza. In A Thousand Plateaus, I would start with the ethology and geology, the refrain, the smooth and the striated. I would also dig into his sources (from Spinoza to Pierce and Bergson to complexity theory) and work from them. Of his interpreters, I would recommend Manuel DeLanda (especially A Thousand Years of Nonlinear History), Bonta & Protevi, Clare Colebrook, Brain Massumi, and the applications to film, music, and the arts (Bogue, Buchanan, Grosz, et al).
But I also like the way his thinking has rippled in so many directions, reviving Spinoza (among others) in productive ways, setting off eddies and flows around the notions of affect (which brings together feeling, thinking, embodiment, subjectivity, and the presocial), ethology (which brings together humans, animals, and environments), ontology, territoriality/territorialization, production, etc. — into political theory (via William Connolly and Hardt/Negri), cultural theory and art & film & music practice, science studies (via actor-network/assemblage theory) and belatedly into environmental theory (via Jane Bennett, Stephen Muecke, Bonta/Protevi, Connolly, Guattari’s ecological activism, and see rhizomes 15 for some other starting points).