Design
Being a web designer is one of those professions where you have to balance objective requirements with your own subjective feelings about want you want the Web to be like. Because of my education in philosophy, I'm get to sit back and watch as certain web designers attempt to portray their own subjective opinions about how a website should be designed as objective fact.
Browser Capabilities
Screen Resolutions
Web Design Software
XHTML, HTML, and CSS
Web Standards
Search Engines
There are really only three search engines to be concerned with; four if you really push it. Basically, anyone building a website should be concerned with Google, Yahoo, MSN, maybe Ask.com, and in that order of importance.
Funny thing is, with the differences in the algorithms that each search engine uses to rank websites, it is very rare that you will see a site doing well in all three of the big ones at the same time. A site that ranks high in Google might also rank high in MSN, but will probably be doing terrible in Yahoo. Likewise, the sites I see at the top of Yahoo almost are a. There was a time when I could promote my sites equally in all three at the same time, but it seems the current trend is that webmasters are better off focusing on only one or two of the big three at a time.