One of the recent trends I have been seeing is the idea of additional functionality for donors-only. This raises several questions for me, most important of which is Is this really a donation? There are a couple of different aspects and implementations that have me divided on this topic.
Ad-funded / Donations
Some of the applications out there, such as Titanium Backup, Rom Manager, and many many more offer a free version of an application with advertising embedded, and a non-free version that removes the advertising. The important thing to note with most of these applications is that they grant identical functionality to both paid and non-paid users. In this case, I see any payments here as an optional donation, because being a non-paying user doesn't lose anything, you only gain advertising, which is trivial to ignore. The correlation I draw is to things like Google or XDA, where they exist for free with full functionality, but I'm presented with Advertisements to support the further development of the free service.
Donations for Functionality
These types of applications have only started cropping up in the past year. In several cases, it looks like these are an evolution of the previously mentioned model. What went wrong, however, was that the owner was unhappy with the donation conversion ratio. In these cases, the company started adding features only for the donation version of the software. Two examples of this include Subsonic, and Mellenaire (a Minecraft Mod). With Subsonic, users cannot stream video unless they have made a donation. In Mellenaire, donators receive the ability to automatically download the right binary in the installer.
With Subsonic, I feel that these restrictions go beyond mere optional donation, and turns the software into a paid product. I have no issue paying for a service or product, but calling it and marketing it as "donation-only" is dishonest, and probably an attempt to avoid regulation, taxation, etc. With the Mellenaire example, it's definitely on the border. From one perspective, the core application (the Minecraft Mod) functions the same with or without donation. On the other hand, users who don't pay the creator lose functionality that would be useful and save time.
What to do about it?
If you see an application or service being provided that removes functionality for non-paying users, reach out to the creator and start a dialogue, and share your concerns that the payment is no longer a donation, but a purchase. Ask questions about the money transfer, assuming it's a purchase. This would include asking about the return policy, any warranties or guaranties, and upgrade policy. Do not let "donation" marketed payments stop you from having the same expectations you would have for a purchase.