As well as an SD card slot I would formerly have also said a headphone socket. But that was because on old phone using Bluetooth was a real battery killer. Not a problem with my current phone and I now prefer the faff-free wireless headphones/earbuds though I still carry a USB adapter in my key pouch along with some wired earbuds as a backup in case I run out of battery on the wireless headphones.
But that does prompt me to suggest that one biggy Android could improve on is giving the user greater/finer control over which Bluetooth device is connected to which phone or tablet and not just allow a device to grab the connection willy-nilly. I have to be careful with just my phone and tablet with a house full of Bluetooth radios/speakers. Goodness knows how a family all with their own phones battle with each other for connection to the living room speaker etc. I struggle at work with a yard full of vans and drivers which all want to be connected to each other at some time or another. I have to un-pair all but the van I am using that day and have to keep killing connections from other drivers until I get out of the yard and away on my own.