The most important thing to look for in all kinds of baby monitors is audio quality. Regardless of whether you want a video-based baby monitor or not, you need clear audio so you can hear your baby properly. You'll also want one with sound activation so that you don't have to listen to white noise 90% of the time. With sound activation, you'll only hear the noises from your baby's room when there's something important to hear.
Most connected baby monitors are effectively just home security cameras, like the Nest Cam Indoor—devices that let you watch another location with color video, night vision, and sound, so you can tell if anything is amiss. Because baby monitors are used to keep an eye on your little one rather than on your home and property, they prioritize different features than security cameras.