I generally shoot 14 bit lossless compressed on my D300 because I usually edit very little. I might change my RAW settings in Capture NX and adjust levels but usually no more than that. Would using uncompressed 14 bit files be worth the extra megabytes added to the file?
From the age of "12-bit" Nikon lossy compression, it meant that something like 653 distinct values were used in the image; the shadows were kept literally, and the midtones and highlights were quantized. Generally, it would not be visible, because there is just too much noise in the captures to be able to have the smooth gradations that an increased number of levels could theoretically allow. I am not sure, however, if they are equal in the highlight clipping points - I've read people said they were the same, and others that the highlights were compromised in lossy mode.
I have no idea what it means in the context of 14 bits. It could mean something as simple 653 x 4 levels recorded, or it may still be 653 levels, but with the slower, more deliberate and cleaner "14-bit" readout within the camera. It seems that the D300 reads the sensor very differently in 14-bit and 12-bit modes; it's not just a matter of dropping two bits for 12-bit mode. It may be called 14-bit also because when it is decoded by a converter, it is returned to 14-bit precision, which, in reality, is probably the only good thing about 14-bit data per se. The Pentax K10D and K20D cameras are the only DSLRs in production that really need more than 12 significant bits of storage (or it's equivalent in the shadows) at ISO 100 (only), and they don't have it! Everything is backwards in this world.
I would think that even if there are only 653 levels used, 14-bit lossy is probably better than 12-bit non-lossy, especially in the shadows, because the 12-bit readout is faster and sloppier.
Ideally, I'd want the 14-bit-style readout, 12 bits of significant storage ("lossy" only in the loss of the two bits of mostly noise), and 16 bits of precision into the converter, but I don't think that such an option is available.