I'll start with my experience/thoughts and edit in additional findings. (Note: I'm bootloader locked, and now unrooted.)
- The UI is definitely smoother. It doesn't jitter or stall under heavy usage anywhere near as much as it did in Jellybean.
- Battery life when the phone is asleep seems to have been improved a little.
- USB audio out is now supported.
- The occasional "screen jitter" that plagued the Jellybean build is gone.
- KitKat's Quick Settings panel is quite handy.
- The native lock screen now supports adding, removing, and rearranging widgets.
- Although it can't be installed from the Play Store, the Google Experience Launcher can be sideloaded and works just fine.
The Neutral / Very Subjective
- The AOSP keyboard has changed; the keys are smaller and, in landscape, very squashed. You may need to adjust to it (I'm trying, but it's a struggle) or find a replacement.
- Battery life when the phone is actively in use has not been noticeably improved in my experience; but it hasn't been noticeably worsened, either.
- There's now a shortcut to the camera from the lock screen, which can be used even if the phone is locked by a pattern/PIN/etc. (Great if you're a shutterbug; not so great if someone else decides to troll your Dropbox/Drive account.)
- The Google Camera app works - but Photosphere can't be used, as the Razr M doesn't have a gyroscope.
- No more root, for those with locked BLs (me). (Saferoot fails, and many users are reporting losing root even when they try survival methods.)
- Wi-Fi tethering via most apps (e.g., FoxFi) has been blocked, so now you must pay your carrier for the feature unless a new workaround is discovered.
- All notifications I've seen so far now flash the notification light white, instead of unique colors. This may be frustrating for some who used the colors to decide whether to check the phone immediately or later. (Luckily, there are ways to address this.)
- The Blur launcher is same 'ol, same 'ol. It uses a solid black background for the soft keys and notifications bar (rather than taking advantage of KitKat's transparency support), and it still has that Quick Settings menu on the left (which is redundant now). In addition, Holo Launcher (which I was using) doesn't account for the space used by the softkeys at all, making it difficult to use. (I've switched to Nova Launcher, which is working fine.)
- I've experienced some weirdness with the application switcher. Sometimes it drops me to the launcher right after displaying the list, and sometimes apps will overwrite another's position on the list, or be dropped from the list entirely without reason (this is always temporary, however - just bring up the switcher again).
- Some apps can no longer write to the SD card - or at least not wherever they please. We still have the Files browser, mercifully (but I favor Total Commander anyway, which seems to work perfectly).
- The audio quick switch on the lock screen has been removed. Also, the buttons for entering a PIN are smaller.
- (Leraeniesh) Under heavy usage, the Razr M may get hotter under KitKat than it did under Jellybean, and may drain the battery more.
- (GPIX) Some users have noticed decreased network performance in some apps, such as Chrome or YouTube, while on Wi-Fi and with Wi-Fi power-saving enabled.
- (GPIX) Several users have reported popping noises - when a sound begins to play, or in the middle of playback. (I haven't experienced it, however.)
- Swype is no longer included. If you used it in Jellybean, you will need to purchase it on the Play Store after taking the OTA.
- (ls3c6) Some have encountered problems with sharing the address book to paired devices over Bluetooth.