Occupancy rates. That makes it hard to do two things - book on short notice (which for some room types can be 'anything less than eleven months out and even then you may not get it - see BLT standard view and VAKL concierge and value) and to do ordinary maintenance on rooms - sometimes things that would take a room out of service for a day or two get delayed - refurbs can get scheduled (in my opinion, its often enough, but the rooms DO look worn before they get refurbed), its harder with the "someone smoked in the room" type of maintenance.
A disadvantage to me is that it really ties you to Disney. That is why a lot of people bought, but plan on using your WDW points at Disney. The times of year where you'd want to be at HHI or VB or Hawaii - and year round in California - are hard to book. Now, if you want to spend January at HHI, you probably won't have any issues.
A disadvantage for some people - not for me - is that we don't qualify for bargains on points. While DVCers will often say that "free dining" isn't much of a deal, it really has been a good deal in some years. When Disney needs to raise hotel occupancy, they have a lot of levers their marketing department can pull to make the hotels look very attractive.
As a trading timeshare, DVC isn't a good deal. Its expensive, its options are limited and members don't have direct access.
It ties you to Disney over the long term. For me, my peak Disney experience was in the late 1990s. The food was good to excellent, the parks were clean, the crowds were reasonable. It is possible Disney will start to ramp up its game to reach that again, but for the most part we've noticed a slow but steady degradation in the things we used to love. For us, food is a huge one - its gone from good to excellent to occasionally inedible to once in a great while surprisingly good. When I pay $100 for a meal (we eat signature, because over the past decade the one credit restaurants have been inedible too often) I don't want to be surprised that the food is good. Now, I'll freely admit that we are a family of food snobs (well, my son is a fifteen year old boy with fifteen year old boy tastes - but even he can tell a good steak from a bad one) and for us, vacations are in part an excuse to eat out and eat well. That is becoming harder and harder at Disney.
Another "ties you to the long term" issue has been inflation. Not only has food gotten a lot more expensive, park tickets have. And, outside Disney's control, but still something we are tied into spending, the cost of airfare.
The beds are not good. They aren't bad, but I wake up with a backache at DVC. My husband is worse. (I like a hard bed, he likes a hard bed and is more sensitive to a divot in the bed than I am).
On the plus side:
We LOVE having the kids sleep in a different room. This is an affordable way to do that.
On property this is one of the FEW ways to do that in anything approaching an affordable fashion. As the kids have gotten older, being on property means my son or daughter can grab a bus on their own - they can sleep in, or they can head back.
We - against my expectation - love having a washer and dryer right there. We come home with clean laundry and it is so easy to pop in a load every night, put it in the dryer every morning, fluff and fold it every evening. Likewise, the kitchen has been a boon to breakfast. We don't use it for other meals, but its great for breakfast.