1. Don't trust the client. Some day someone malicious will try to give you invalid input to see what will happen.
2. A significant and growing fraction of users have some form of pop-up blocker installed. Assume that any pop-up will not be seen.
3. Older users use low resolutions even on monitors that support high resolution. Any web designer who assumes that the general public has the eyesight of a 20 year old should be fired. With prejudice.
4. Users understand user interfaces differently than programmers do. What is obvious to you will never be figured out by them. If you don't have a UI specialist, then you need to learn something about good UI design because you will get it wrong.

The key principle, to me, is to fail gracefully. Use all the bells and whistles you like, but make sure your core functionality is accessible to users without all that. Say the low income blind person(1) or the curmudgeonly old-school geek using Lynx. Or the security-conscious user with all client-side scripting disabled.

It's a little more design effort, but it's not that hard. If you can't implement a particular function without the extra stuff, perhaps you need to re-think that feature. ("Do X in one click" is not, for purposes of this argument, a feature. "Do X" is.)

(1) There are screen readers that work with GUIs. They're expensive.

Personally I try to avoid using javascript, and have javascript disabled, as much as possible. Yet, until perlscript, or some new invention, is as widely usable as javascript, I will continue to need it for various applications. The only reason I meantioned using perl was that I prefer perl to check variable tainting and the if statements could be changed to run a sub that will do something in javascript.