We've all been there. Not knowing what to play from 100s of games and when you build up quite a backlog it's also easy to forget about games you actually own. This is the point of Play Next. Originally a Steam Labs experiment, then entering the Steam Client in a recent Beta and now it's out for everyone in a stable release.
If you have unplayed or games with "very low" playtime, there's now a Play Next shelf in your Steam Library.
While I see it as quite a useful feature, as I know plenty of people who pick up a big list of games during sales, it's also easy to see why people may not like it. Another feature nagging you and it might even perhaps feel a little embarrassing that you have so many unplayed games. Thankfully this can be moved or removed if you wish as it's just a Shelf.
This stable release of the Steam Client also came with other improvements. For Linux, Valve fixed the "registry.vdf" file sometimes becoming corrupt after a forced system reboot and issues with cloud sync and game downloads when ipv6 is completely disabled in the OS. Another Linux fix is for the Steam Cloud, to have Steam use the correct local relative path for game data for some games on Linux. Steam Overlay should also be more stable with OpenGL, as a crash was solved were in rare cases after a screen resolution change while the overlay is not up.