I've never had the temerity to ask for an upgrade -- just for a particular location within the category I booked, and I'd say those have been granted about 3/4 of the time. Anecdotally, friends in hospitality tell me that people ask for free upgrades with such regularity (and in many cases, with such rudeness and/or a sense of entitlement) that they really don't give them to anyone on request anymore, unless it's a VIP or someone they have reason to want to do a favor for. (Case in point: my brother-in-law happened to strike up a conversation with a CM when he was checking into a standard room at Art of Animation a few years back, after noticing that the CM's home town on their name tag was the same small town in central NY that he had grown up in. It turned out they knew tons of the same people, and they had a good time reminiscing. By the end of the conversation, the CM offered to switch them to a family suite - for no additional charge - the following day, when a bunch of them were expected to become available. That's how my sister's family ended up with a huge upgrade!)
The last time we visited WDW, we received an unsolicited upgrade, from a garden view room to a beautiful water/fireworks view at Yacht Club. It may have had something to do with our celebration of our 20th wedding anniversary, which was marked on the reservation -- but then again, we've had all sorts of other celebrations on various reservations over the years (anniversaries, birthdays, etc.), and never got an upgrade before, so it may have been just a coincidence. If you are celebrating something, I would recommend noting it on the reservation or making mention of it in your room request: perhaps someone will be moved to pixie dust you!