I really like this idea, and have long thought of doing something similar, the reason being that most/all existing services that I've come across so far don't do a good job at it. They can actually be even more depressing than normal news (to me).

One reason reason is that the "positive news" are often reported against something really depressing, don't point to sustainable solutions and so defeats the purpose. In essence I believe "positive news" must either turn a slightly blind eye to the vast number of problems facing humanity (without being essentially just pictures of cats), or mainly bring up such problems if a true solution has been found (not "eating more raisins may reduce cancers in mice").

If this problem could be solved (by heavy manual curation?) I believe a service like this could actually take off.