I get 4 hours compared to 5 under OS 9, and the list goes something like this.
- Display brightness
- Audio (mute audio will cut power to audio hardware)
- Hard disk sleep (OS 9 had option to sleep manually, hence slightly better)
- RAM disk in OS 9 helped reduce access to hard disk and could let you push out batt life to 6 hours plus.
- Peripherals (such as mice, USB powered devices)
- Processor speed under OS 9 could be reduced when on battery power.
- OS 9 had a consumption meter, so you could tell how changes were affecting your battery usage.
I don't notice this with my iBook. My battery lasts a little over 4 hours with both OS 9 and 10.
One main reason the battery may not last as long in OS X is because OS X needs more power to run. The genie effect, dock magnification, quartz, all use the processor heavily. You didn't have that with OS 9.