1) Hyphenation tables (required for iBooks; this is something that’d be deep in the guts of the code, for devs only, and unlikely to be openly mentioned).
2) App switching — people cite Expose, but I expect a surprise here. Pressing the Home button twice, as rumored, seems tacky. I really think they’d want a software solution for this.
3) Would they also preview a new iTunes? It’s needed for desktop reading of eBooks and for syncing book notes and for syncing Pages revisions (a feature which needs to be added).
4) More syncing options with MobileMe. Users of the abominable Kindle say they’d like to have similar iBooks syncing — that is, read on iPhone on the go, have iPad sync your spot when you get home.
5) What if Apple brought back something like HyperCard? Now wouldn’t that be a nice kick in the face to all the Over-IQed Wingnuts? Something like that could even be used to create digital books! (If Apple did something like this, how fast would we see all the whiners change to whining about how “anyone” could now create things instead of needing them and their expertise?)