To begin with, your criteria can be as broad or narrow as you like. You can name a song, playlist or genre, or ask to play any kind of music if you're not picky. Alexa can stream radio channels from the likes of TuneIn and iHeartRadio. Naturally, there are a few perks if you use one of Amazon's music services. You can ask Alexa to wake you based on a mood (like "relaxing"), or find a wake-up song by reciting the lyrics.
This sounds like a minor feature, but it's potentially very important. If Amazon is going to make the Echo Spot a viable alarm clock, it needs to give the device better functionality than that 20-year-old clock radio sitting on your nightstand. This also makes all Echo models more directly competitive with rivals that have had music wake features for years, such as Sonos. And let's face it: even if you're just using Alexa on your phone, Amazon would rather be the one to start your day.