What information does a wallet contain?
From looking at the code, the wallet.dat can contain entries of the following types:


name - an address book name
tx - a transaction
acentry - an accounting entry
key - a pair of public and private keys
mkey - a master key
ckey - an encrypted key
defaultkey - the default receiving address' key
pool - a keypool entry
version - the version of the software that wrote this wallet
minversion - the earliest version of the software that can read this wallet
cscript - a script
bestblock - a pointer to the end of the best blockchain seen
wkey - a private key (no longer used)
setting - used to store user interface settings (no longer used, except to hold an invalid setting to stop pre-0.4.0 versions of the client which don't understand minversion from loading wallets that are too new for them)


See also this answer.