Joel Spolsky – How Trello is different:

“The great horizontal killer applications are actually just fancy data structures.

Spreadsheets are not just tools for doing "what-if" analysis. They provide a specific data structure: a table. Most Excel users never enter a formula. […]

Word processors are not just tools for writing books, reports, and letters. They provide a specific data structure: lines of text which automatically wrap and split into pages.

PowerPoint is not just a tool for making boring meetings. It provides a specific data structure: an array of full-screen images.”