The biggest user-facing change in Android 4.4 KitKat is, without a doubt, the launcher. The new launcher experience provides deeper Google Now integration (it's literally the leftmost homescreen), beautiful transparent navigation buttons and notification bar, always-on Google Now listening, and a much cleaner app drawer. For now, though, this launcher will remain a Nexus 5 exclusive - Google wants to see what the reaction is before expanding this 'Google experience' to other devices or the Play Store-using public.
Personally, I'm really liking the new launcher. It makes Google Now much more accessible, and I'm definitely using it a lot more than I was before. The aesthetic changes are obviously a matter of personal taste, but I'm a big fan of the larger icons and transparent nav buttons / notification bar, too. I used custom launchers extensively in the past, but I find most of the features they offer don't really excite me anymore - I'm not that concerned with fine-tuning the launcher experience on my phone these days. So, if you have a Nexus 5, what are you using? I've decided not to provide options for individual third party launchers, as there are just so many. Instead, I suggest you head down to the comments and find a response for your choice of launcher, or if it's not there, leave one for others to vote up.