A player name must be a single word, must not contain any spaces, backslashes, or quotes, nor can it begin with the characters #, *, (, or ). Finally it cannot be one that is in use by any other player nor any of the words on the following list:
E_VERBNF
@@@ Helpfile alert: Previous line is messed up; notify a wizard. @@@
Note that these rules apply as well to your single-word aliases, since those can equally well be used to refer to you in commands that match on player names (@who, whereis, ...). There are no restrictions on your multi-word aliases, however the commands that expect player names will not recognize them.