But the biggest difference is that nobody really knew what we were doing. The thing YC and others have brought to the industry is a consistent path for startups to follow. If you want to start something, and want to go down that high-risk path, there are playbooks to follow and incubators to apply to, with easy access to information on how it will look.
None of that existed back then. Nobody was walking around talking about the pros and cons of VC vs. bootstrapping, or how to get funding, or how to avoid getting scammed. The basic practices of developing a product, finding product/market fit, iterating based on feedback... those weren't common knowledge.
Now lets also get rid of stack overflow, blogs, and pretty much most sources of information online -- all of that came later. You bought books and read them. You asked your friends for info, and hopefully they knew something. Usenet was a thing, and that is where you might find better info.
Lets get rid of the Agile manifesto, too -- people were either doing waterfall or winging it with their own processes. And the jobs were mostly consulting gigs, requiring lots of meeting and proposals, not self-service signups to a SaaS.
Let us not forget that we did not have git, either. We used old-school source control (if any), self-hosted, check-in-check-out style workflows.
In short, we were (mostly) all inexperienced beginners, without access to much information, flailing about trying to do something new, using really old practices. It didn't really work. But it was kinda fun.
Our technology was novel and patented, but relied on the cooperation of ISPs, which we never got. (It was basically mass statistical analysis, but sourced and shared across ISPs. That model lost to huge free webmail providers instead.)
Because anti spam wasn't making money, the founder decided to sell the technology to the Chinese government instead, to use to censor dissidents. I had a long, angry chat with him and resigned that same day.
I use the heuristic that if there are loads of billboards everywhere advertising stuff that should be incomprehensible to the layman, then things have got too frothy.
Fast forward 5 years though, and one of my first full time jobs was being a contractor for a small e-commerce company that (barely) survived the dotcom bubble.
They had a lease for a full floor on a high rise, but were down to a skeleton crew when I was brought in. I got to pick any office I wanted, since there were so few workers, so naturally I picked a corner office with an ocean view.
I thought life was great since I was getting paid decent money, I was in my young twenties, and the company was pretty chill about me just adding some features here and there and keeping things running.
The other folks though would speak about the 'good times' when they'd throw lavish parties, when the boss would hand out $100 bills to all the employees, etc. Sounded wild.
I quit before the bubble burst, but, later, that year I purchased a dotcom business at a rock-bottom price, and turned it into a great job for the next 20 years.