I would really like to see a confirmation page added to the building upgrade button.
I'm one of those people who sometimes accidentally hits upgrade when I really just want to visit. You'd think after having made that mistake a few times, I'd be careful enough not to do it again. But when it's the wee hours of the morning and I'm getting over-tired, that's when it happens. Imagine finally getting a product system running perfectly and then when you mean to activate your last building, you accidentally click upgrade to level 9. Yup, that's what I did last time. Doh!