Do you restart your client often? The debug.log is just a logfile that is recording events that your client considers notable, such as restarting. You can delete that without a hickup, and the client will just start another upon the next restart. blk0001.dat is the actual blockchain while blkindex.dat is the file that your client uses to quickly find a particular transaction within all those blocks. I would still recommend that you backup your wallet.dat file before deleting anything, though.
According to my process listing (I'm running this under Debian by the way) the client has been running 179 hours, about 7.5 days when I installed it. I just left it open and running on another desktop. I just closed it out and restarted it and it cleaned up about 800MB of data from the database directory. I opened up the debug.log and it appears to be a listing of all IRC traffic that the client generates. Perhaps the next client release can clean up this file as well when the client shuts down?
The debug log exists for development reasons, and having the client automaticly delete the log upon exit would be self-defeating. I suppose that a startup batch script that destroys the existing debug.log file upon restart wouldn't be too dangerous. Or you could use the -noirc switch to disable the irc bootstrapping altogether. You only need it upon the initial startup of a fresh install, or if you wipe out your database. You don't really need it at all, it just makes finding the ip addresses of the first few connections easier. After that, node discovery is done via the p2p network itself. I always use -noirc. You need to do it if you are using tor or I2P, otherwise the irc code will just leak out your ip address anyway.