Don’t Make Me Think, Revisited a common Sense Approach to Web Usability Steve Krug

clear
and 
informative
and explain exactly what
your site or your organization does.

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 Good taglines are 
just long enough, but not too long
. Six to eight
words seem to be long enough to convey a full thought, but short
enough to absorb easily.
Good taglines 
convey differentiation
and a clear benefit. Jakob
Nielsen has suggested that a really good tagline is one that no one else
in the world could use except you, and I think it’s an excellent way to
look at it.
Bad taglines sound 
generic
.

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NationalGrid can probably get away with using a motto instead of a
differentiating tagline, because they’re a public utility with a captive
audience, so differentiation isn’t an issue.
Don’t confuse a tagline with a motto, like “We bring good things to
life,” “You’re in good hands,” or “To protect and to serve.” A motto
expresses a guiding principle, a goal, or an ideal, but a tagline conveys
a value proposition. Mottoes are lofty and reassuring, but if I don’t
know what the thing is, a motto isn’t going to tell me.
Good taglines are 
personable, lively,
and 
sometimes clever
. Clever is
good, but only if the cleverness helps convey—not obscure—the
benefit.

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Tagline? We don’t need no stinking tagline
Some sites can get by without a tagline. For instance:
The relative handful of sites that have already achieved household
word status.
Sites that are very well known from their offline origins.
Personally, though, I’d argue that even 
these
sites would benefit from a
tagline. After all, no matter how well known you are, why pass up an
unobtrusive chance to tell people why they’re better off at your site? And
even if a site comes from a strong offline brand, the mission online is never
exactly the same and it’s important to explain the difference.