Sundial prevents attacks of creatures. This prevents almost everything
-No ability uses (permanent or creature) -- they can still be used with sundial
-No spells --not stopped by sundial and silence only affects the enemy not both players
-No attacks of creatures OR weapons (sundial only stops creatures)
Essentially it is supposed to be like skipping a turn for each player except for drawing, quantum gain / upkeep and non-interactive effects like poison and freeze duration countown.
In terms of game play this should actually be quite a difference, primarily because with sundial you can still play creature abilities and spells. So, for instance, if your enemy had a growth creature you wanted to kill with poison, this would make it much easier.
-You can also use it to ride out timed effects more effectively, e.g. phase shields, cloaks, etc. Since the other player cant attack or use spells every other round. (effectively cuts their duration in half)
-It gives a good counter to nova and Immo rush decks which are often produce less quanta from pillars. (they can only burst every other round while you gain quanta every round)
-"Fat" decks get a little extra time to hunt for other cards and have a better chance to force a win by deck out.