After alt.* was inagurated, the very first group was (IIRC) alt.swedish.chef.bork.bork.bork. Then there were alt.destroy.the.earth and alt.pave.the.earth, which were bitter enemies despite their opposition to alt.save.the.earth, because if you destroyed the earth there'd be nothing left to pave. Alt was also the home of the Big Seven sex groups, plus piracy in the alt.binaries groups. The talk.* groups had problems as well. Oh, and then there was alt.2600, which purposely had a moderator who rejected any and all postings: you had to hack USENET and assume moderator privileges to post there.
I think easily the closest thing we have nowadays to USENET is reddit. And, to be honest, if you average over its entire audience, it's more tame than USENET was.