When I had my G1, I would easily jot down things when they occurred to me, and could admin servers without half the screen being obscured - I can't do those with current devices.
What I've never been able to do is quickly prototype ideas to check their feasibility, or simply run scripts and applications written in any language I wanted.
I noticed that Planet Computers (the company behind the Gemini/Cosmo) are planning a 5G slider device for next year - likely to be popular amongst people who don't like clamshell but still want a keyboard...
https://www.indiegogo.com/projects/astro-slide-5g-transformer
:-|