From the su man page here's the explanation for the "-" :
su [ - ] [ username [ arg ] ]
- Pass the environment along unchanged, as if the user actually logged in as the specified user.
username The name of another username that you wish to log in as.
arg Additional arguments that need to be passed through the su command.
So basically, the shell environment is different with the - as opposed to the naked su which allows you to execute with root permissions but retains your original user shell environment.
As for the launch icon, it would depend on the patch level of the version of kde 4 that suse uses. In a lot of the earlier versions of kde4 that many distros used, kdesu was broken; that's why it won't work.