Complimentary upgrades are usually only given by necessity. Such as when a guest needs non-smoking and there are none left in that category. It can be due to party size, special needs, etc. Another reason for free upgrades is when the resort wants to sell out and has lots of higher category rooms available, so they sell more lower category rooms than what they have in order to sell out and then they start upgrading people. Lots of guests try to get upgrades at the GF and are usually told that they will need to pay for them. It does happen on occasion that a free upgrade will be given without necessity, but it is not as common as you think. For all of you loyal Disney guests, please don't feel slighted for not getting an upgrade. It's not that the cast members don't want to give you one, they most likely just can't. Please remember though, that front desk cashiers and room assigners get yelled at a lot! No matter what you have heard, yelling, threatening and complaining does not help you get a good room.
In other words, it's just the luck of the draw. So, if you want a specific room, my advice is to book it.