The main reason you’re going to need a baby monitor is to answer a simple, but time-honored, question: Why the hell is my baby crying? It’s only been in the last 30 years or so that parents have relied on the remote surveillance of their sleeping children. For the eons before that, it was a combination of natural, ear-piercing cries, and sleeping in the same yurt.
For standalone monitors, we found that two features were nice to have: The first is VOX, which mutes the ambient sounds in a room entirely, activating the speaker (and often the video) only when significant sound is recognized. The second feature we appreciated was LED sound meters. These are an excellent, rare feature to keep constant tabs on a baby while muting a monitor and turning the video off (Consumer Reports appreciates these LEDs, too.)
The features we focused on were those we thought either increased the performance of the monitor or made it more user-friendly for parents and increased the odds of getting good quality sleep. We looked for monitors that have sound activation that keeps the parent unit quiet when the baby isn't crying, so parents can potentially fall asleep faster because they don't have to listen to white noise. Some of the monitors were so loud, even at low volumes, that the white noise might keep light sleeping parents awake; this defeats the purpose of having a video product to begin with. We also liked the models with screens that automatically "wake" and/or go to sleep.