I agree with comments from Shannon, Ellen and Ian about the model changing and problems contributed by AirBnB, among other issues. We as Slow Travelers have all been using vacation rentals for years and the effects weren't negative back in the day; but as pointed out, we rented for several days to a week or more, they were a longer-stay alternative to hotels, which also have their place. Having spent many many years in the tourism industry, I see lots of changes and negative impacts of these unsustainable models.
I have been a slow traveler and vacation villa user for many years, but I don't (and won't) use AirB&B; it went from being a "couch surfing" type of thing to a major deal-breaker for communities in many ways.
1) As someone pointed out, there are investors now buying up apartments solely to rent them on a grand scale through this platform, raising prices for locals in both the purchase and rental markets; locals forced out of their own cities bleeds out the lifeblood of the place;
2) Many rentals aren't "legit" in terms of registering and paying taxes, putting those who play by the rules at a disadvantage (Giulia, Letizia and others who rent holiday apartments as examples); (I read recently that Venice is one city that is starting to crack down on this, looking at the listings and knocking on doors and tracking down owners);
3) short stays in this type of structure (1-3 nights) has an effect on the hotels/B&Bs and thereby the local economy, as hotels and B&Bs employ people whose jobs get put at risk;
4) local agencies who broker legimitate rental properties also proviode customer service, guarantees and assurances, will resolve problems, inspect the properties to ensure they exist and are as represented, and of course they also employ people to provide customer service and administrative support;
5) AirB&B provides no actual customer service and is a listing site that "double dips", taking commission from the owners AND from the travelers (in exchange for what?);
6) what used to be an alternative has exploded, and now entire buildings in the some cities have turned over to short-term rentals, forcing out residents who give the city its character and charm, stripping out that local flavor and appeal (I know a person in Rome who moved out the historic center because every apartment in her building became a rental and the constant flow of noisy tourists who were disrespectful of residents with noise, trash and leaving the street door open had her fed up). Cities devoid of their own residents become touristy shells and much less appealing.
As for mass tourism, it's the absolute worst thing a place can have. Meaning, tour busses and tons of cruise ships, and tons of day-trippers. It is not sustainable; this type of tourism brings only crowds that overtax a community without contributing to the local economy; they arrive, disgorge, look around, maybe buy a gelato and leave. Huge crowds tramp around, follow their guide, get back on their bus - very little money is left behind to deal with the mess they make on the instrastructure, and they don't pay the so-called "tourist tax" that those who do overnight stays must pay (which is also a bit erroneous, taxing those who are staying longer and spending money in places that provide employment rather than levying higher fees on the fly-overs, but...that's another discussion). Cruises could be limited; busses can also be limited and should be paying fees for each load, etc. to relieve some of the massive crowding.
But it also falls on each of us to choose how/where/to whom we spend our travel dollars and do so as responsibly as possible to benefit the communities rather than further stress the infrastructures of a place. As at least a small part of the solution.