Actually 300mm on a 2x is not. Minimum 400mm on a 2x sensor. I do primarily birds. I have thought numerous times about the 100-300, but I haven't seen the detail out of that lens that would warrant buying it. If Pany had the same sharpness on the 100-300 as they do on the 35-100, I'd go for that, but still it falls short for what I do.
Also, for stills I'd rather have something with a longer reach. Cropping a cropped sensor leads to more noise, but primarily I plan to use the GH4 for 4k video and stills are secondary.
Andrew, if you could see the incredible clarity of using the GH3 with the Canon 600mm f/4 lens, you'd want one too. I use this combo all the time but yearn for IS and aperture control.