Ability to scan bar codes.
Ability to enter ISBN.
Integrate with calendar.
Integrate with to do list.
Send email instead of SMS/Twitter.
Ability to have default due date (at my library, books are 3 weeks, CDs/DVDs are 1 week; for books I'd want 3 days, for discs I'd want 2 days).
Different default alarm date for different types.
And, as always, the fewer the taps it takes to do something, the better. Try to figure out ways to make it as efficient as possible. Ideally:
1. Launch app
2. Scan barcode(s)
3. Tap OK at any point to accept defaults for all items.
Optionally, for those with access to multiple libraries, would be a way to pick which library (e.g. scan library card's bar code first, tap on list (not a pick list unless more than 3 libraries please!). Of course, if no library is defined, or only 1, don't make it a step to pick which library, the way it is now is fine :)