Hi, shamelessly plugging our own app here, but we just released a new app called "AppVegas" that helps with exactly the problem of finding new apps that are getting good reviews.
On the home page, there is a section for "new & trending apps" which is exactly what you are looking for.
Additionally, you can browse each individual marketplace category, and at the top of each list, you get 1 row dedicated to the new & trending apps for that category, as well as 1 row for the all-time highest rated apps for that category (that feature is more for users new to the phone).
If you would like to try it, the link is: http://www.windowsphone.com/s?appId=...5-43247567e681
Lastly, on the very first pivot of the homepage, there is a slot machine which you can use for finding apps. If you tap the "lock" button above that middle wheel and lock "Hot" into place, it will find you apps that are relatively new and that are getting awesome ratings.
Similarly, you can lock "New" in place, and it will find you apps that are brand new but unproven in regards to their reviews/ratings.
If you do try it out, let us know what you think!