There are slumlords - but that's usually caused by a compression in supply (e.g., when I was in college, any place near school had notorious landlords). When supply is open though, everything works out.
You have your uber-cheap rentals where nothing will ever get fixed, and then you have your more premium rentals where they'll turnaround a stove in 24 hours or less. Like all markets, a low-end and high-end are established and work just peachy.
Of course, this is not geographically true everywhere - especially when you introduce artificial supply and demand factors (rent control, anyone?).
In the case of a landlord, they are often representing their own brand, and their income is dependent on the success of that brand. In the narrow scope of a single contract, there is incentive to defect to the point of barely-tolerable, but a landlord is not acting on behalf of the renter.
Additionally, in a world with a greatly increased population of renters, there would likely arise a more organized market for landlords. I think you'd see something a lot like what we see with restaurants. The larger brands have a very specific reputation that they reliably follow, and the smaller chains and unique locals rely on word of mouth and overdelivering. Then it's a short jump to a sort of Yelp for landlords - in fact, a lot of places have this already.
Related: back in school, the student legal union kept track of all complaints filed and who the landlord was, then made the log available to anyone. The distribution was as expected: Campus Property Management, the largest and most commonly detested landlord, had the most complaints and the highest complaints/property managed. Bill, who managed the house I lived in senior year along with a few other places, had none, and he was awesome.