That's a big one for me - if I want weatherproof and mirrorless, Olympus (and therefore M43) is really the only option. I don't trust Panasonic weather sealing (mostly due to absolutely zero experience with it, to be fair), Sony 'weatherproofing' is a complete joke, Canon and Nikon systems are too new to really trust, and my experience with Fuji weatherproofing is that it's nowhere near as good as it claims to be.And yet the only company I can trust to make an English (weather) proof camera is Olympus (in mirrorless form, else Pentax would take that crown)
If you want a camera to shoot in bad weather, Olympus and Pentax really are the top dogs by quite a large margin.