If you want fast updates, then you need to get a Nexus device or one of the developer phones.
As far as KitKat and going from Nexus 4 to Nexus 5, there are several changes. I won't talk about the phone changes since you wouldn't care about that, but instead focus on OS changes. This will pertain mostly to DEV devices as Samsung puts their own BS bloatware crap on it. Don't miss Samsung and Touchwiz at all!
Anyway, with KitKat there's now a preview version of ART (Android Run Time) which replaces the Dalvik VM. It will be the standard runtime in Android 4.5 when that ships. Basically, what it does is pre-compiles all of the apps into native code and improves startup times and battery life. Keep in mind, Android runs ANYWHERE and to accomplish this, you need a virtual machine (VM). I can type more, but this gives you an idea.
The screen is now visibly as large as it is, since they've made the notification bar and navigation transparent. Nice touch. Previously, in the notification area, the white icons would turn blue when connected. Now they're just a light gray silhouette and turn white when connected. Meh, not a fan. You used to have up/down arrows on the WiFi signal or on the mobile signal to tell you when it's sending data/receiving data. That's gone. Don't know why, but I miss that.
People say there were performance improvements, I didn't notice any. Jelly Bean was already fast. There are efficiency/battery life improvements (even without switching to ART).
Quick list:
- There are more camera options, more editing options of pictures
- Can take picture with volume up button (is that new? I saw my wife doing that recently on her N5)
- The icons are now much larger
- Quickoffice is now included as part of the OS (uses local storage or Google Drive)
- Alarm & location are now a toggle in the settings drop down (two finger down or swipe down notifications and click settings)
- Printer drivers are now installed so you can print to your local HP printer or do "cloud print" (whatever that is, I haven't used it)
- Google Wallet is better integrated
- Has VPN support (don't remember if the old one did)
- The dialer has been completely changed (some good and some bad here, but mostly good, just need to get used to it). If you're calling your contacts it's just slightly different, but now google search is built into the dialer so if you start typing in a local business it'll give you a list of numbers - very cool feature
- Widgets are now no longer part of the app drawer (FINALLY)... they're accessed by long-pressing on the screen
- Google Now look like it has a few updated cards (not sure if those are available to everyone now though - see below)
- Location based services now have a better way of grabbing location instead of always invoking the GPS when you have it on (helps that everything is better location aware and doesn't drain the battery)
- In general, Google seem to be more location and time aware. In the morning when I go to my bus it has all the traffic info as it used to before, but since it asked me if I take the bus to work, it now has my bus schedule, time of arrival and all the other bus routes near by along with times/duration to get to my destination (again, awesome)
- Google Wallet is obviously location aware. I was buying something at Walgreens a few days ago and it automatically pulled up a Walmart rewards card, etc. Little touches like that.
- Keep app has notes, time/location based reminders and other crap... but what's interesting is that Google Now has its own reminders when you say "Ok Google, Remind me to walk the dog tomorrow morning" or something like that.. don't get why they're not together
- SMS is now part of the Hangouts app, so no need for two shortcuts now and Hangouts is heavily revised. Much nicer and you can archive conversations
- No more "Navigation" link... nav is just part of maps and that also has some improvements that I noticed after upgrading, but for the life of me can't remember what they were
- Desktops are now dynamic... add/remove as you like, reposition, etc. Add a desktop by just putting something on it if a desktop is empty it's automatically removed. The home screen is now the first screen, no longer the "middle of 5" screen
- Clock app is much easier to use; a lot more features, easier to set alarms, etc (I don't remember everything I had on the old clock, so hard to comment on specific changes, I just remember it being better)
- Obviously everyone is aware that Google Now is on every desktop and accessible through voice... "Ok Google"
- I used to be able to add widgets to the lock screen before (besides camera) and now I don't know if they removed it or I don't know how to add them, but not there (not a feature I used, just something I knew existed)
- Much more Google+ integration, but really just mainly for your local phone info and contacts
I'm sure that's not everything and not very detailed, but this post is already too long. Not sure if this helps you or not...
