Gilles Deleuze is a Continental Philosopher of the first rank his major work beingDifference and Repetition. He tried to develop a philosophy that was purely immanent with no transcendentals at the Wild Being meta-level of Being. Thus he is in my opinion the true successor to Merleau-Ponty who identified Wild Being as a possibility, which is the dual and next level up from Hyper Being (the hyper dialectic between Process Being of Heidegger and Nothingness of Sartre) which Derrida called Differance and Heidegger called Being crossed out. Creating a philosophy at the Wild Being level is very difficult because it is almost unthinkable. The strategy of Deleuze was to take earlier outlier philosophers and to rethink them in radical ways. Zizek is very insulting towards Deleuze in his commentary on this strategy of Deleuze. The strategy is related to the strategy of Foucault in The Order of Things of taking secondary works instead of the pivotal works in philosophy as the measure of the change in Epistemes. Deleuze is trying to do something similar by taking as his point of departure philosophers that have fallen out of the mainstream and using their voces to make his own points, which is ironic because his central concept is the univocity of Being. He is trying to take these various outliers and make them speak with one voice, i.e. his own. Much of his philosophy is a reaction to Lacan, so that is why Zizek does not like him. Deleuze takes Lacan’s ideas and reworks them to make sense. Lacan himself is almost unreadable and many of his ideas do not seem to make much sense taken along with everything else he says. Lacan is arrogant and obscure on purpose. His philosophy is a form of DADAism or Surrealism. So Zizek’s reworking of it to make some sense is useful. Deleuze tried to take certain peices of Lacan’s implied philosophy and make sense of it, for instance he tried to make sense of the floating signifier in the Logic of Sense which is the true successor to Russell’s theory of higher logical types which is the source of the meta-levels of Being when applied to the concept of Being.
In Difference and Repetition you learn toward the end that repetition means “that which cannot be repeated”. It is a philosophy of the meta-levels of difference making that seems to celebrate heterogeneity, except for the crucial passages where he talks about the univocity of Being and then you see that behind the scenes is a Monism as Badiou pointed out. So Deleuze’s philosophy tends to be quite nihilistic, even more so than Derrida, and that is because Deleuze has gone on to the Wild Being meta-level of Being from the Hyper Being level and so there is an intensification of Being. Badiou was extremely influenced by Deleuze and tries to correct his implicit monism by developing the idea of the Multiple and the Event as additions to the concept that Being is really just Set theory. The Multiple is suppose to be pure heterogeneity before the arising of the One (ultra-one) with no monism behind the scenes. Event is the opposite of repetition, it is the emergent event that changes everything. So if we read Badiou as a reaction to Deleuze, trying to take Deleuze deeper then we see the importance of the thought of Deleuze. What Badiou substitues for the Monism of the Univocality of Being is Mathematics which in all its variety is very stable, and thus the opposite of the Event that changes everything. However, even though Badiou is trying to radicalize the already very radical Deleuze and treat him as a jumping off point for his own work in Being and Event, we see that the actual philosophical practice of Deleuze aligns better with Wild Being. Badiou is monistic in his practice even though he is trying to bring true heterogeneity to the fore without implicit monism. Deleuze uses all kinds of things as his jumping off points going in many directions in his lines of flight that lead to intensifications of various sorts producing a very rhizomatic ouvre.