Listen to the geeks
Computer programmers rely on the Internet for their work - it is the only place for many of the tools they need. I myself am severely impeded without a network connection. This reliance on the Net gives programmers a unique perspective on how to make things usable; as they use the Net more than anyone else, they know how to make processes and common actions as efficient as possible.
Listen to the newbies
New users, on the other hand, may have little or no technological background. They can't be expected to read manuals; they may not even have access to manuals. For that reason, the design goal is to make your software intuitive to the point where anyone can learn it on the fly.
Just because everyone else does it ...
...means you have to do it too. People expect things to work in a certain way (i.e. so it becomes intuitive). If your application doesn't work like everyone else's, people may not understand it, or worse yet, think it is broken. Pay attention to how other sites handle forms, logins, and the like and make sure your interface is equivalent.
The need for speed
The greatest threat to good usability comes from poor download speeds. A slow download breaks a user's train of thought. In a business environment it's pure inefficiency. In a consumer environment it's lost sales. The guidelines I like to use are as follows:
Now, some might say that those numbers are outdated. After all, broadband is here, isn't it? Not quite. The majority of Internet users are on a dialup. More importantly, the advent of new technologies and networks goes in two directions - large and fast or small and slow. Good usability dictates that we satisfy both extremes.
When in doubt, use Google as a guideline. The size of their index page is carefully and deliberately calculated.