If you are afraid of the "FULL DESTRUCTIVE CAPABILITIES" you can connect as user instead of root. In fact I would highly recommend doing this as saving stuff to places like MyDocs as root can cause you all worlds of permissions headaches.

However it is slightly more complicated than simply connecting as root, as you need to use x terminal to set the password for user as by default it does not have one. You also need to be root so install rootsh from App manager before you start.

HOW TO SET A PASSWORD FOR USER:
Everything in quotes should be typed MINUS the quotes then press enter afterwards.

Now once rootsh is installed simply open x terminal (pressing Ctrl+Shift+X is the quickest way I find) then type "sudo gainroot". Now type "passwd user" and you will be asked for a new password for user, follow the instructions given. Once done if you get "Password changed" then all went well so you can just close x terminal now, if not then try again, you may have made a typo. If in doubt, close x terminal and start from scratch. The N900 will happily work even if you do not know the password for user and root can always change the user password without you needing to know what it currently is, so there is no worry of getting it wrong (as long as you do not mess up your root password when installing OpenSSH that is).

Once done, follow the instructions as in the first post but instead using user instead of root, and of course the user password you just set. This is far far safer as you cannot damage the root filesystem as user plus the permissions will correctly be set as user for any files you copy over to your N900.

DISCLAIMER:
I have not noticed any negative effect of setting a user password on my N900 but as the OS by default expects no password for user, I cannot guarantee something strange wont happen especially after a firmware update. But then you ALWAYS make a backup before a firmware update anyway, don't you!? Of course as long as your N900 boots correctly, you can always remove the password again later by doing the same thing but just leaving the password blank, so it would take a major cockup to cause a problem.