MOTD's
MOTD stands for "Message Of the Day". In Sven-Coop and other HL1 based mods they are used as an introduction or briefing for a map. For Counter-Strike, Half-Life Deathmatch and other "Official" mods you can put HTML in the MOTD which makes life a lot easier as MOTD can slow down things at the start of a map.
As far as I know the text based MOTD's have a maximum of around 250 characters. Occasionaly if you uses weird centering and slash ("/") characters you could have problems.
Many players get "reliable channel overflow" and or sz_getspace errors if the MOTD is too large or configured in a way that the program doesn't like.
Conventional wisdom on many of the admin boards is that server admins are better off removing the motd's and putting them in a seperate folder for reference. Most players just click thru them anyway and it's not worth taking the chance that the MOTD is going to crash players.
My conclusion for mappers is that if you are going to make a MOTD, make it short and simple.