The 9780 is much different than the 8800. Many of the improvements, i.e. 3G, better screen, etc., require more battery.
Battery life depends upon how much you use it and how many data intensive apps are running int he back ground. Anything more than a business day should be considered excellent.
It actually helps a Lithium battery to be charged and topped up. Letting it run down below 20% can actually harm the battery. I keep my 9800 plugged in when I am in the office.
This link from our FAQ may help: Maximum Battery Life - BlackBerryFAQ