Especially with things like the Twitter leak, I've become increasingly frustrated with the fact that someone could basically just become me if they a) found out

Especially with things like the Twitter leak, I've become increasingly frustrated with the fact that someone could basically just become me if they a) found out my Gmail password or b) had access to a pre-authenticated session (e.g. my flatmate borrowing my laptop). The fact that the Gmail password in and of itself also ties access to other Google services, as pointed out in the OP, doesn't scare me half as much.