This article on the landlord website Property118.com shows a simple example, demonstrating that a private landlord with a mortgage could see a huge jump in their effective tax rate (in this case, from 18% to 67% by 2020), while a corporate landlord will see no change at all.
There's also a link in that article to a detailed report which is highly critical of the tax changes. The government obviously take a different view! (See here for more worked examples of how the tax changes will be applied).
More information can be found on this on various landlord sites. A key phrase to look for is "section 24", referring to the section of the Finance (No. 2) Act 2015 which implements the change.
Note that this change only applies to private landlords (i.e. those who own a property personally, rather than through a company), and who have a mortgage on the property, and who (after the new calculations) are higher or additional rate taxpayers.