Big surprise: more features require more power leading to shorter play time.
Call it PSP deja vu, but if it's really a big issue, buy another battery.
Better sound, better screen; not a bad trade off not even factoring the other hardware additions.
On the other hand, deleting the GBA slot is kind of a breaker if you have a lot of carts, not that I had immediate intentions of buying a DSi anyway. GBA is a dying/dead platform anyway.
I presume the real reason for deleting the slot is so that Nintendo can begin selling old GBA games through the Nintendo Shopping Channel, just like the PSP/PSN Store.