Maintenance fees/taxes vary somewhat by resort, but you can just estimate they'll be around $4.00 per point. That is the only annual expense you'll ever have.
There are absolutely no expenses when you check-in and use your points. There's no sales tax, no room tax, no service charge or anything like that.
You can opt to have daily housekeeping service which is an extra charge. You can also get extra towels, toiletries which also would be an extra charge. If you don't opt for any of this then you have no charges.
Only other expense would be telephone charges if you make phone calls.
Your room key can be used as a charge within WDW to purchase merchandise, meals, etc, just like any hotel.
