For one, there would be no Nazi party and the Holocaust as we know it (industrial genocide, gas chambers, death camps) may not have happened. Nazism was not at all inevitable: in 1920 it was just one of several populist parties that attacked the order of Versailles as illegitimate. It was Hitler and his charisma that made the NSDAP a major party; he had both the oratorical skill of a populist and the ability to cozy up to the Weimar Republic's military-industrial elite that dominated the country. Unless someone like Hitler comes out of nowhere, the German Workers' Party (not even the National Socialist German Workers' Party; that was a result of Hitler's little coup against preexisting party leaders) is likely going to remain on the fringe of German politics.
Communism would not prosper, however: there was nothing Hitler could do that would allow the KPD to survive, and Germany's elites made it very clear that any kind of pan-leftist coalition that included the Communists would be forcibly dismembered. I see the Weimar Republic formally dragging on for a few more years - remember, Germany was already more-or-less a dictatorship before Hitler, with all of the previous Chancellors only having Presidential (and not popular) support - before finally collapsing. I find it likely, actually, that a conservative, aristocratic (maybe monarchist?) regime could take over Germany; led by members of the Army and the landowning Junkers, it might try to recreate the 1916-18 military dictatorship.
Does that mean WW2 is averted? No. These people were very revanchist, without all of the pseudoscientific stuff. A German reclamation of its 'rightful' lands in the East were not viewed as a hypothetical; all that was ever in dispute was when they ought to be claimed. In 1922 a German minister was making public statements saying that the conflict between Germany and Poland was "a struggle" for "Germandom and Kultur [German civilization]" --- and this is a member of the center-left Social Democratic Party saying this! The power vacuum in Eastern & Central Europe after WWI was also too large, making war inevitable -- as Germany concurrently sought more land in the 1930s, revanchist sentiment was also high in Hungary, Romania, Italy, etc. The second World War will likely happen later, as the German military elite who argued against Hitler's expansionist policy (on the grounds Germany was unprepared for a large-scale war against GB & France) and were discredited by early German victories will now be in control of the country.