I'm a video noob. And I'm curious... What's wrong with shooting at 60fps?
I understand how ND filters work with aperture, but if slo-mo is what you want, why would you shoot at 30fps with an ND filter, rather than 60fps without one?
Nothing wrong with shooting at 60fps, in fact it's often used so that in post-production you cut the output framerate in half to get 30fps output so you double the time it takes, which give slow-motion.
Beyond that, the general rule in video is to get a 180-degree shutter which gives a nice, smooth look you shoot at 1/(shutter speed * 2). So for 30fps you shoot with a shutter of 1/60, and with 60fps you shoot at 1/120, or as close to that as you can get (so 1/125 on most DSLRs). So the only way to control exposure is with ISO or aperture, or to use ND filters to cut the amount of light. Since the OP wanted a very shallow DoF he needed to use some ND filters to cut the light to allow his aperture to be wider since he undoubtedly had his ISO set very low (100-200). Since he had no ND filters, he had to raise the shutter speed significantly. This can give video a more stilted look, since every frame is much 'sharper' and there won't be any motion blur like you can sometimes get with the slower shutter speeds.
Very well explained! Thank you. I'm saving up for a variable ND filter.