For those who don't own $DIS, the 'advantages' that many who are looking forward to it appear to be:
1) The ability to make FP+ reservations 60 (or 60+10) days in advance
2) The 'personalization' that may occur on some experiences
3) The magicband itself
Why these 'advantages' look like they won't be positive (in most cases) after all:
1) 60(+10) - First off, the dining ADR is 180 + 10. A person who likes to eat a lot of Disney Table Service will be making their ADR's well before 60 days prior to arrival. This will have the effect of 'locking in' their itinerary. Then when 60 days comes, they may find that the parks they are eating at don't have any FP+ for what they consider to be 'good' rides that day.
Second, ride breakdowns and the potential for your reserved ride to 'change' on you. Under the 'legacy' system, a FP that is for a ride under a temporary closure is 'rainchecked'. The new system may not be able to handle this, and may reassign you last minute to something that you don't want (and possibly too close to that rides time to change it - so that Space Mtn fastpass turns into a Stitch FP at the very last minute). (And if they use the 'airplane' model in reserving these, you might be 'bumped' from your time, not because of a ride malfunction, but because too many 11:05's walked through the main gate and they have to reassign 'less magical' guests).
Third, the FP+ limit. A person who knows how to get the most out of legacy fastpass can get their hands on a respectable amount in a day. Get 1st FP. When 1st time comes up, first get 2nd FP, then do 1st FP ride, lather, rinse, repeat. A person could potentially spend a marathon day at the MK on a Saturday (easily the busiest day of the week) and still manage to get a respectable amount of things done. Now there's a 3 'experience' limit. (sounds like from last testing the 'categories' of rides may not be a part of it).
Fourth, the 'one park only'. A benefit many of us enjoy is the 'park hop'. Which isn't always a ride in the Magical Motorcoach. It can be a ride in the Monorail. Or a pleasant walk through the Boardwalk away from the crowds. And with 'legacy', when you get to the next park, some 'hyper-popular' attractions may be out of FP, but others may have some to turn 20-30 minute waits to 5 minute ones. Not anymore.
Fifth, the affect on lines. More FP+ attractions doesn't add capacity (a point many, many have made). So a lot, lot more attractions will have longer standby lines. A ride that is 10 minute wait w/o FP+, w/ FP+, if half the people wait 2 minutes, the other half will *at least* now wait 18 (could the 'bottleneck' affect make the standby waits even more than 'averages' say they should be?).
Finally, the affect on guests around you. There are plenty of 'unhappy' people there - families yelling at each other, etc. People in more of a rush to get to their FP+ reservations (which may not blend too well with ADR reservations) pushing through others - some of those rushing in ECV's that they don't fully understand how to use properly. And in the 'other' parks, alcohol is more likely to be in the mix...
2) personalization - will there be an option embedded in case little Johnny would be freaked out if a talking foamhead or face character knows his name? And if they decide to add this to rides, what happens if you tell little Joey that the dolls at the end of Small World will know his name, and they name - little Cindy instead?
3) the magicband itself - there's a reason they 'prefer' you to use it on your wrist instead of a RFID card. Better tracking of where you (and your minor children) are. Not just when you enter a park, what FP you use, and what you buy on your room charge - but everywhere you go (what stuff you look at in a store, what 'sponsored' rides you ride, etc) - creating a valuable database for them to directly SPAM you with (and share with their 'partners' to do the same - perhaps with the sponsors to each and every ride). And don't forget the 'minor children' part. Their response to Markey's COPPA inquiry suggests that this is something that they see as a huge value to them and are willing to fight the government to keep (instead of saying that they will just collect on the 14+ crowd). Perhaps sneaking through a loophole, but totally violating the spirit of a law designed to keep children safe from unwanted electronic communications.