> Does the fact that a lens has a single focal point get in the way of keeping depth information ?
No, really single lens have separate focal point for each wavelength, but achromatic optical system in SLR have at least two lenses, so they have range of frequencies for which focal points are very close to one designed point.
Mirror optical system don't have chromatic distortions, but in SLR case lens just cheaper to produce.
Lytro using other idea - light field from short distances is not square but rounded (square light field from distant stars, because of very large distance, so radius too large to see rounding), and in Lythro each point seen via prism by few pixels (each angle with other pixel), so could measure distance to light source and using this info, restore distance information.
Unfortunately, Lythro technology means, with few Megapixels CCD, will have just some hundreds Kilo-pixels restored image, so in reality, need some hybrid approach, where will be classic high-resolution 2D and some sort of depth sensor. Plus, need additional processing power to calculate restored image.
So yes, in reality, latest Lythro camera using huge CCD and very powerful processor, all too expensive for COTS (but acceptable in some cinema niches).
Anybody could make Lythro-like setup from ordinary lens and CCD (and make calculations on for example Raspberry), but now Lythro patents prohibited to make money on this.