Everything. We're down to barebones and marketing now focus solely on camera software updates or phone materials ("now with titanium!" How fucking sad is that?) And they are all selling the same phone.
Some of the most important loses...
Swappable batteries changed travel for me. Always having two extra charged batteries in my backpack, that you could swap top 100% in 20 seconds, made me ONLY use my phone as a free and completely useful tool without any planning or restrictions on my use. Otherwise, you can't take too many pictures or videos, stream music or video or make video calls too long or you might be fucked when you need phone, GPS, payment or to get a rideshare to where you're staying.
Audio jack similarly meant freedom. Bluetooth headphones out of battery, broken or one earbud lost? Have a pair of wired in the backpack always add backup. Also better audio quality through wired with DAC on certain models and less daily device load to charge/babysit
secondary screens LG V10 had a bar on top, they also had the T shaped dual screen phone and the secondary screen phone case. There was just creativity and attempts at innovation.
microSD expandable memory, again less and less available and this was about freedom - fuck your cloud storage add its data leaks, corruption and redaction. I own my data, you don't control it.