CG galleries, missing items, etc. always do a good job of compelling me to continue, but unless I know something about them to make me care about the content more than the goal (like knowing that a locked CG is part of an alternate root for a character I really like, or the item has a description that is intriguing, or seeing an interesting silhouette).
However, I think the most interesting system is a route map
like what School Days did. This didn't occur in the game, but having the arc names above each arc is a great idea too. The map doesn't tell you what choices will lead you where, it just tells you where the forks are.
I also like it when new dialogue is clearly marked (using a different color after the first playthrough is pretty common) - especially when a scene is similar, but a little different. It turns me off when I'm stuck reading a long scene that sounds identical to what I read last time, but skip is disabled because some minor detail changed. On the flip side, when changes are clearly marked it makes me want to re-read the scene so I can get the full effect.