While I think the 90's was a pretty good decade, I can think of various reasons why is was bad.
Music: Good - It was the decade of Grunge, which was the final death nail to Glam metal and West Coast Gangster rap became more mainstream. Bad - Grunge was short lived due to Kurt Cobain's suicide, the West Coast & East Coast rap war became too serious and it took the lives of Tupac and Biggie. Bubblegum pop soared thanks to Brintey Spears and boy bands like N'sync and Backstreet boys. Also, Vanilla Ice, Rico Suave, and MC Hammer stayed for too long. Woodstock 99 was terrible.
TV: Good - Sort of MTV's golden era. Thanks to Beavis and Butthead, TRL, Headbangers Ball, and Yo MTV Raps. Kids cartoons were great thanks to Nicktoons, Fox Kids cartoons, and Kids WB. That, and the last few years of the decade brought the Anime boom from Cartoon Network. Saturday Night Live was fantastic. Was also the decade we saw the rise of South Park, The Critic, and King of the Hill. Tales from the Crypt. The "Stone Cold era" of WWF. Xena and Hercules were there. We also had Friends, Fresh Prince of Bell Air, and Senfield. Bad - 90210, Power Rangers went on for too long until Pokemon killed it's fame, MTV started their way into reality tv dumbness with Real World, Daria, Fox Kids got rid of many of their best cartoons too soon, Headbangers Ball and Yo MTV Raps got cancelled too soon. Cop Rock. Fox News started. Jerry Springer. Barney and Teletubbies.
Films: Good - Goodfellas, Jurassic Park, Free Willy, Edward Scissorhands, Boyz in the Hood, New Jack City, Dazed and Confused, Silence of the Lambs, The Nightmare Before Christmas, Lawnmower Man, Wayne's World, Reservior Dogs, Reality Bites, Pulp Fiction, Terminator 2, Forest Gump, Toy Story, Braveheart, Fight Club, Titanic, Independence Day, Species, Austin Powers, Armageddon, Total Recall, The Big Lebowski, Blade, There's Something About Mary, Varity Blues, Starship Troopers, Harry Potter and the Sorcerer's Stone, the Matrix, Office Space, Se7en. Bad - Everything else. Especially Godzilla and Phantom Menance if you believe it ruined the franchise.
Politics: Good - Bill Clinton got into office after 12 years of Republican presidency and the Cold War was over. Bad - Newt Gingrich and the Republican controlled House, Monica Lewinski scandal, the rise of Right wing radio hosts like Rush Limbaugh, and the debate over political correctness.
Video games - Playstation, FFVII and FFVIII, N64 and Mario 64, Dreamcast, and Mortal Kombat. NES and Sega Genesis were ok but had their problems. Bad - Arcades became almost non-used, Sega couldn't get their act together, Dreamcast died, the Mortal Kombat controversies and anger over them thanks to Joe Libermann and Jack Thompson.
And of course, that's just a sample. Still, unlike Bob, I quite liked the 90's. Still, there was quite a few things that sucked about it, but then again, almost every decade has something that sucked about it.