Current restrictions
http://www.swtor.com/free/features
Announced changes
Preferred Accounts will have four action bars standard.
Preferred Accounts will have six character slots.
Also worth noting
The restriction on flashpoints does not mean you can't run them, just that you can't roll on more than three last boss drops.
Restrictions can be lifted by purchasing them off the GTN or directly from other players using credits rather than cartel coins if you don't want to use money.