Everyone always talks about how good the 90's were. What was bad about the '90s?

If you didn't know something, you probably had to find a book that contains the information you need. Most of the time you just had to accept that you don't know.
Indoor smoking.
War and massacres in the Balkans and in Africa.
- probably had to find a book that contains the information you need
  that was the case for all of pre-internet history, not just the 90s...
  the 90s was mostly just a continuation and worsening of things that were already there--rampant consumerism, economic inequality, social injustice, environmental destruction-- the "bad things" really can't compare with the 00s and the 10s, or, especially now, the 20s

LGBTQ rights in the US were awful. Sodomy was still illegal across much of the country, as was marriage between two people of the same gender. Blatant homophobia was rampant and dominated the culture. Coming out as gay could ruin someone's career, and coming out as trans was much less common than today, because most people spent their entire lives hiding who they were.

Gods, the bigots.
Not that they're any better now, but they were actually worse in the 90s than the 80s because gay rights in specific were winning, so they just nhad to swing dick more now that they felt like they were losing control

The smell. Folks don't remember but indoor smoking was recently phased out in America and all the restaurants and malls and theaters all smelled like stale tobacco smoke. Clothes and cars had marinated in the smell for years. Only school was clean smelling. It was a nightmare.

You had to choose between a small set of approved groups you could belong to. It was especially bad in my small village in western Germany: you could be either a fashion victim (all with the same hair, clothing style, one year they ALL dyed their hair red), a computer nerd, a sporty person, a hip-hop or a techno fan. If you didn't belong to any of the approved groups, you were an outsider and bullied.
Obviously I'm writing this because my best friend and I were among the outcast. We bought our stuff at thrift shops and didn't care much for fashion, neither what was 'in style' nor the geek/techno/hip-hop fashion. One year I cut my hair really short and got so much shit for it, because apparently you could only do that if you 100% subscribed to a matching Goth aesthetic. (Still have short hair, so take that stupid mean girls!)
It was probably partially a small-town-thing, but if you look at the media from the 90s you'll rarely see characters that don't neatly fit a certain type.

Just off the top of my head:
Matthew Shepard
Rwanda
Ruby Ridge, Waco, Unaboms, and Oklahoma City
Abortion clinic bombings
Bosnia
Rodney King / LA Riots
Don't Ask, Don't Tell and DOMA
Seattle WTO protest violence
Columbine
OJ

It really depends on where you were living - if you were living in former Jugoslavia with the war going on, the 90s were pretty bad. And 2025 in China is better than 1990. Despite everything currently happening, the world is in many parts better than 30 years ago.

I remember watching something really interesting on TV, and they had finally arrived on the topic I was waiting for. And just when they were about to show the answer to something I'd been wondering about for ages, my grandma walked in front of the TV.
I vividly remember that sad sinking feeling as I silently concluded "I guess I'll live the rest of my life without knowing, then..."
- I learned how to program the VCR when i was really young. Then Pokémon came out. I missed the first episode, but i was so happy that I could watch the series, I didn’t care that much. Then, I learned that the series was repeating, and they were showing the first episode!
  I set up my VCR to record it, as I was at school when it played. I got home and dashed to the family TV, and started watching. I was sooooo happy to finally see the episode that started it all!
  During the Spearow horde, Pikachu jumps on Ash’s shoulder. Then the cable cut out. Silence. Until it comes back, and Ask and Pikachu are laying on the floor, coming to grips with what just happened.
  I fell the the floor and WAILED, pounding both fists on the floor. I is maybe the most devastated I’ve ever been in my life.
  
  Damn, that must have been horrible
- You could have gone to the library and looked in the encyclopedias for a sentence fragment on the topic.
- Damn, that must have been so frustrating!

Dog owners felt entitled to leave shit everywhere. Everywhere.
- And the poo turned white after a few days in the sun.
  
  I think they used to put bone meal as a supplement in dog food back then, which is what caused the poop to turn white when it dried.
- Past tense? Definitely improved somewhat, but there's still enough assholes to go around.

Los Angeles Riots were the culmination of continuous LAPD and LASD harassment and criminal behavior towards citizenry. The whole affair was terrible, before, during, and after.
I would say that eventually the city healed but I don't think anybody I know has an ounce of trust in either department.

Baseballschlägerjahre (baseball bat years) in east Germany. If you speak German or want to try your luck with automatic translation, here's a source. Neonazi violence didn't disappear after the 90s, though, and it wasn't only in the former GDR areas.
I think the former GDR areas had a rough time in general - sure they didn't live in a dictatorship anymore, but there was mass unemployment and many who had somewhat better jobs lost their status (e.g. public officials, history teachers, workers in now uncompetitive companies or engineers who were working with suddenly obsolete technologies). The contrast to the rich west Germans made it worse, and many of them bought out east German real estate.
Also, tobacco smoke everywhere all the time.

Having to rewind a cassette tape with a pencil eraser because it got stuck in your Walkman. Broadcast TV was vapid and overrun with ads. People who used email were geeks and social outcasts.

A lot of prejudice that was hidden
- A lot of prejudice that was very much in the open too.

It's a nostalgic feeling, it had good parts and bad parts like any other era. If you were a teenager at that time you probably swear it produced the best ever music. The fall of the soviet union and the end of the cold war gave people hope the new millennium approaching will be a peaceful different one.
And then the wars in the Gulf, Yugoslavia, Bosnia, Afghanistan, Somalia, Congo, Iraq, Yemen, Armenia, Sri Lanka, Ethiopia, Kosovo and others show us we're still in the same shitty world.
- I was a teenager back then and I did not think it produced the best music. But I was not mainstream, I hated being like everyone else, as if we were robots. I made a point of going against the stream regarding aesthetics, religion and cultural stuff. I hate uniforms to this day. I don’t get the young these days dressing the same as the others. Is it an education thing? I mean, as a teenager you are looking for yourself, trying stuff to see what you like. Why would you just copy what everyone does?!
  
  Ironically the 'nonconformist rebel who thinks themselves a more unique individual than their peers because they don't listen to the same music/dress the same way/have a niche hyperobsession/ act antisocially' is one of the most common and popular teenage archetypes.
  Using generic punk identity signifers like dyed hair and piercings and only being into underground non-mainstream artist to show your not like everyone else, only serves to indicate that you are in fact predictably generic in identity seeking like everyone else.
  So I guess my point is that maybe nothings changed about kids, its the same old same old. Most of them follow the trends and normalize to fit into tribal groups, the ones that don't pride themselves on nonconformity while paradoxically adopting tribal markers to distinguish themselves as a group that conforms to nonconformity.