How much info is appropriate to give for Knowledge check for a monster

[danzibr]

In case the title isn't clear, I'm talking about a player using Knowledge (whatever) (http://www.d20srd.org/srd/skills/knowledge.htm), which
allows you to remember a bit of useful information about that monster.

For every 5 points by which your check result exceeds the DC, you recall another piece of useful information.
So my question is... as a DM (or your experience as a player), what counts as a bit of useful information? One thing it's resistant to? Or one thing it's weak to? How about stuff depending on its type? Like if it's a skeleton, it seems it wouldn't be enough to say, ``It's immune to sleep effects.''

[Deophaun]

I go for what would make the creature notable. A basilisk is notable for being able to turn people to stone with its gaze (DC 16), not so much for having good fortitude saves and a thick hide (DC 21), and its visual acuity in the dark is widely unremarkable and so rarely remarked upon (DC 26).

[Flickerdart]

Later Monster Manuals give specific tables for what can be learned about a monster with specific DCs. So you'll see things like "DC X reveals the qualities of the undead type" which is the first thing anyone would know about skeletons. "Oh, it's undead, so it's immune to all this living people stuff" is one chunk of info.