Of course not being able to plug in USB sticks, mice, keyboards, or anything really due to lack of USB-A port is a significantly huge hassle and a rather big step back for "it just works" motto of previous laptops :/

USB-C flash drives are already out there, and cheap. Will still need the dongle for transferring between the new Macs and non-USB-C devices for ~3 years.

These are laptops with built-in keyboards and trackpads. People who opt for external input devices do so via Bluetooth. I've never seen a USB keyboard plugged into a MacBook; and even if you have a fancy gaming keyboard, USB-C replacements will be out in a matter of months. Or again, the dongle.

This series of laptops is a perfect progression. It needed to have 4 USB-C ports, and it does. No complaints.

Well, other than the lack of a hard escape key. Turning shift+F6 into shift+fn+F6 is annoying, but I'll get over it. The only applications that I use fn keys with have key remapping, so if I really can't stand it I'll change my shortcuts. The escape key though... not looking forward to the day I upgrade my hardware and find myself having to enable the "caps-lock is now the escape key" preference.

Bunch of generalisation. Plenty of people use external devices via USB with Macbooks. Now all of them will need adaptors. Same goes for people who have a bunch of flash drives. New ones may not be expensive, but it's still a waste of money when nothing actually stopped working.

Agreed. Accessory makers aren't going to build the ecosystem if they don't have to, and most manufacturers are apparently too chicken to commit to type C (let alone TB3).

So the installed base of projectors is going to sprout a usb-c connector in the next two years?

It's no worse than the current situation, where most projectors only take VGA. If you have a MacBook Air or older MacBook Pro, you already had to carry a Mini-DispalyPort adapter regardless.

If you own a projector, you're used to keeping adapters around for different computers. If you give presentations frequently, you're used to keeping adapters in your travel bag for different projectors.