The gist of the Adorno-Benjamin beef was that Adorno thought Benjamin played too fast and loose with the idea of the Marxist commodity. The way that Benjamin describes the commodity in The Arcades Project, Adorno felt, did not sufficiently reflect what was going on with 19th-century consumerism and consciousness. Adorno thought that Benjamin needed to get a little more forceful with his Marxist argument. Benjamin's whole fragmented, dreamy, and mythical way of describing history just didn't have enough social reality for Adorno.
Adorno offered some of this criticism in really nit-picky remarks in the margins of Arcades—like a fastidious grammar teacher with a red pen. And these two also wrote some heavy correspondence debating these issues, but Benjamin's suicide ended the exchange of letters. Ultimately, Adorno wrote an essay about Benjamin's opinions on mediation and representation, which he apparently took great exception with. Curious? It's called "On the Fetish Character of Music and the Regression of Listening."