According to the wiki with sighted sources
Known Expert Mode Options
friendly fire from area of effect spells and abillities.
A character dies permanently if he has 0 health, if this option is turned off he becomes Maimed instead.
hide which Attributes and Skills are checked for options in conversations and scripted interactions.
show just the journal text of an quest in the journal, not the journal text and an explicite quest objective.
hide unqualified conversation options.
hide the reputation and disposition effect of an option.
hide the disposition values in the character sheet.
hide companion influence messages.
hide Defense and Accuracy tooltips.
hide higlighting from area of effect spells/abillities.
show only brief System feedback.
That last one links to a page that reads, "Brief vs verbose system feedback." My guess is this will range widely as well including showing dice rolls to not even showing target values. I like the idea of hiding them in expert mode. I too like solving the mystery of when I could hit something.
