The first thing I'd like to bring up is bloatware. Unnecessary programs being pre-installed to your system that you'll probably never use and don't care for. Examples of these include carrier backup/cloud storage, carrier tv, and sports applications. These can't be uninstalled and sometimes can't even be disabled unless you have root. Which, speaking of..
I Am Root
Rooting is the ability to have full administrative access to your system. It's also a complete pain for devices that have locked bootloaders. Which, your bootloader could be locked from a few different groups. Whether it's through your device manufacturer, the specific model of the device, or through your carrier, it makes owning the device annoying. This is because you can't fully control your device through rooting and can't do what you want with it. Which could include installing a program ( like AdAway ) to block ads across the device ( without using the VPN interface like NetGuard does ) or having a fine-tuned iptables-based firewall with AFWall+. With root, you completely own the device and can do whatever you want with it. Well let me rephrase, not exactly anything..
There are Quite a lot of permissions on Android. This in theory is a great idea but in practice falls short by a large amount. One such issue is that access to the clipboard is not restricted by any permission. This means that if you copy something from one application meant for another, it could be intercepted. There's also no simple way to verify this has occured. Luckily there's a program which allows you to prevent this as well as fake a lot of information you give to programs. That program being XPrivacyLua ( requires root, not affiliated ). I personally use it and find it incredibly useful. Feel free to check it out on Github: https://github.com/M66B/XPrivacyLua#restrictions.
CAN YOU HEAR ME?
You can't set volumes on a per-application basis through Android natively! Why is this important? If for example you had a popup window of YouTube Red and another program that made sound, the first will stop and the second will play. This is even more complicated when some programs don't even give you the option to change the volume at all on it! There's a program called App Volume Control Pro that claims to fix this problem. Though, seeing as the company is called "SpyCorp", I'm not sure if I would personally trust installing a program by them.
Who needs updates?
Last thing I'd like to touch on is updates hardly happen on anything except expensive flagship phones like the Google Pixel line ( even then, you only get 2 years of updates ). Buying an Android device should guarantee you at least a year's worth of monthly updates, yet most devices will hardly give you two updates if you're lucky.
I'd personally like to see Android do much better in regards to all of this. There's obviously a lot that needs to be done, and all we can do is wait and hope it all gets fixed.
Questions/Comments/Issues? Feel free to contact me using the methods below.