Most commercial applications allow you to try them in one way or another before buying, if I'm not allowed to do that (which by itself smells bad) commercial applications more often have reviews (at least user reviews) and if I'm ultimately about to buy an application that I haven't tried or really got a feel for you can bet that it is because I'm becoming desperate and haven't found anything else decent (in other words I've put in quite some time in exploring alternatives before even considering to buy that application).

The same goes for this software model, only when I buy an application I usually get some support etc. If I donate, I usually, already have what I'm donating for. Quite a big difference.

And honestly, if the author is deliberately trying to make my life difficult just to trick me into donating I'm going to be annoyed, and since I deeply disagree with that type of model (donations should be earned, it's not a form of annoy-ware) I will, out of principle, be very hesitant of donating (I'd like to say that I'd never donate, but if the application was truly exceptional I'd probably donate anyway (but I'd attach a comment describing that I dislike it)).

I can put up with quite a bit of tinkering to get something to run, but not if it is because the author have deliberately kept the make-file to himself (as he implied in the comments) just to get people to donate. Huge difference.

I'd probably be more recipient towards a commercial application with a decent trial-version. I much rather pay for something than getting it for free from someone being rude about it.