UX top notch
Missing saving fonts
For the next couple of features I would suggest that you implement login so that you can save fonts in some "user collection" on his profile (preferably with images that he used to test the fonts with). Then in the next iteration you can introduce premium font's, you contact the guys that sell their fonts and you can charge them something (probably monthly fee) so you can promote them on your website. Lastly you could introduce a "bundled download" so that you can download all your liked fonts at once.