I've had this Nexus 5 for a year now, and also still got a Dell Streak somewhere that was used for a few months before being chucked in the cupboard.
The things that bug me about it
* Battery life is worse, even after disabling NFC, removing apps that use location services a lot, preventing apps from using backgound WiFi data a lot. Get 2.5 days on average.
* The "press button to show all your apps". You know the hated Windows 8, with a start screen, and a "down" button to get to a damn ugly, unusable long flat list of EVERY PROGRAM, HELP FILE and accessory? Remind you of something?
* I had about 140 contacts. Went on hols to my sister a few weeks ago, on the way there at a stop got phone out to call her. Not in address book? It only had 90 or so contacts in it? 50 had 'gone' (no they weren't just Skype or Facebook contacts) Oddly the "quick link" to my sister WAS still there, but as it was a link to absolutely nothing (not even a blank contact entry), pressing it did absolutely nothing, confusing as hell.
* The addressbook / contact list is a bloody nightmare. If you log into facebook you do not get a choice if you want it to sync FB contact info, it just does it. Making loads of duplicate contacts, contacts that you only have in FB that have NO info in them as the FB user hasn't shared info. Can't turn off FB sync without removing FB from accounts. Ditto for any damned thing it adds to "Accounts" (apart from Skype which is the only one that DOES give you an option to easily enable/disable contact sync - proving it can be done)
* Updated it to 4.4.3 a couple of days ago wondering if that could cure some of these problems. It may have, but the "cannot use camera" and, "bluetooth stopped working" errors I saw yesterday are not a good sign all has gone well.
* Android games are poor. Trying to find a decent action, racing, flight sim sim on Android is fruitless, there aren't anything comparable to what you can get on Apple. I've no idea WHY this is, as the hardware has been upto it for years but the game writers just don't port the games across. Fav is Radiant Defense, a static 2D shoot-em-up.
Apple is a very closed in ecosystem, I know the app writers tear their hair out sometimes trying to get app approval and having to jump through stupid Apple requirements, BUT the OS and UI still pees on everything else out there from a great height.
(I've got a Windows phone too that was tried for a month - very simple and easy OS and it all worked just fine, but far too restrictive for me)