- Was too hard to test and get working across multiple browsers
- Was just so confusing! Gah!
YUI is the Yahoo User Interface library. It's main strength is as a toolkit of ready-made user interfaces based on common UI design patterns. It also provides cross-browser conveniences for interacting with the DOM, browser events, etc. The documentation is outstanding and it's a very robust system that allows for highly complex and rich interaction in a very short amount of time.
What does this have to do with my startup? Because it helps you build awesome stuff for your customers faster. These tools are client-side frameworks much like there are server-side frameworks. Frameworks are friends.