In an ideal world...
Justifying the statement "this is crap" with hard evidence as to why, and then congratulating the individual on what they might have done best is all you need to make good criticism. I don't see a need to get around the fact that their work is crap with flowery language.
What you're really saying is that you have the rewards system of a child. The application the author wrote is very simple and has no use - what kind of feedback is she looking for?
Additionally, phrases like "What you're really saying is that you have the rewards system of a child" are exactly what I'm talking about. That statement is intentionally worded as an attack, so I have to make a conscious effort to not get defensive. If you want an actual discussion about how rewards systems may change as people grow up, we can talk about that. That was an extremely ineffective way of starting the conversation, though.
It's very easy to couch constructive criticism in a few words of sincere praise/recognition of their work, and it generally results in them doing better work in the future. In fact, they'll be much more likely to listen to your criticism if you haven't immediately put them on the defensive. Once either side gets at all defensive, it's extremely difficult to have any kind meaningful conversation. If you always say "this is crap", then you're literally asking them to defend themselves.
If you haven't already, I would recommend reading "How to Win Friends and Influence People" by Dale Carnegie.