I really just needed some change, because for me it's all about the design. I still like Metro, but I prefer Google's take on flat design these days, as Metro got a bit stale (it's been 4 years after all). I've been saying it since I had my Galaxy Nexus 2 years ago. With every OS update since 4.1, Google's design got better and better (with the exception of the terrible lock screen in 4.3). It started with Google Now and it's slowly getting everywhere. They got rid of the ugly Tron Holo theme, have little animations with great detail, it's bright, colorful and fast and fluid. What they've shown with Android L just confirms it for me and I'm really pumped to get 5.0 on my phone, when it's out.
On a system level it's nice to have little things like actionable notifications (replying right from the lock screen is sweet) and it's good to know that WhatApp and its notifications works reliable 99% of the time. The overall quality of the apps might be better, but I can't really comment on that. All I do is browsing GAF, using WhatsApp and listening to music (and even on Android, TuneIn Radio occasionally drops the connection out of nowhere, so it's not a WP thing).
All that, and the bright neon red Nexus 5 is beautiful.
What's good is that Microsoft is going to mix up Metro for next year, so I might as well be rocking a WP next year again, after I getting bored of Android.
I'm running ART as well and the only thing that has crashed on me so far was the Google Search app. But as I mentioned, I don't really use apps.