Well my global takeaways are that: in general you get what you pay for. The specifics vary between brands and investment levels. But in general the background noise (except on PCs not usually heard directly... not shhhh but the noise floor... allowing you to hear deeply into the music) drops as investment increases. The amount of detail can be good in budget gear, but the overall representation of the audio spectrum gets more continuous. By that I mean on really cheap gear you get treble and bass and little fleshed out in between... thin sounding. Hardening of treble also disappears with better quality streamers. The rhythm and pace, typically absent in budget gear gets better and better except in designs attempting maximum detail.
If you want to look at the two things that most consistently are the biggest thing it is the noise floor and natural nature of the sound. The noise is suppressed through highly refined power conditioning, physical isolation, and vibration suppression. Hence, high quality streamers are heavy... all this stuff weighs a lot. Also, lots of folks attempt to make budget streamer sound good by adding linear power supplies or playing with their routers, and network infrastructure. A, high quality streamer will clean up the incoming signal without the need to clean up your entire network and incoming power supply. The second is the how natural the sound is. The DAC is critical here as well... so if your DAC isn’t great, then a great streamer is not going to shine through.