From livejournal/cgi-bin/ljlib.pl:

# <LJFUNC>
# name: LJ::canonical_username
# des:
# info:
# args: user
# returns: the canonical username given, or blank if the username is not well-formed
# </LJFUNC>
sub canonical_username
{
    my $user = shift;
    if ($user =~ /^\s*([\w\-]{1,15})\s*$/) {
        $user = lc($1);
        $user =~ s/-/_/g;
        return $user;
    }
    return "";  # not a good username.
}

In other words, it can take [a-z0-9_], with no default restrictions. There are, however, reserved usernames, which are configurable, and differ from installation to installation. I don't know what all of them are on LJ, but the defaults are (from ljconfig.pl):

# list of regular expressions matching usernames that people can't have.
@PROTECTED_USERNAMES = ("^ex_", "^lj_");

In other words, it can't begin with "ex_" or "lj_".