Is a Web tagline a branding statement, an advertising slogan or a description of what your site offers?
I vote for #3.
If it's clever, it can enhance your brand and add personality to your site. If it's clear, it tells a visitor where she has landed and what your site can do for her.
Clear is more important than clever
A Web tagline is a tiny but key piece of site usability. A good tag captures a visitor's attention and interest long enough for her to decide if she is in the right place. This makes it a first step in the sales process.
An ineffective tag leaves your visitor guessing what you do. It makes work for him or her--not a good way to introduce how your product or service solves a problem.
Note: Your tagline is the short phrase that appears at the top of every page of your site, usually next to the name of your company and often embedded in a graphic that is part of the page template.
I asked usability guru Jakob Nielsen to comment on the tension between "clear" and "clever" in crafting a good tagline. Here's how he responded: