With Nikon at least, recently I've been disappointed with the range of DX-specific lenses available. It seems Nikon are treating DX more as a second class citizen. When you look at the range of lenses available in DX and compare it to their FX range, the DX ones tend to be slower, more plasticky, and more zooms than primes - in other words, more targeted at amateurs. There are of course exceptions like the good (and expensive) 17-55, but there are no fast normal primes or wide primes in DX-only, apart from one 35mm which is not a particularly good performer for a prime.
It's fine if you only want kit lenses or inexpensive superzooms but this got me thinking about how with Nikon, I'd have to move up at least to FX lenses, if not FX all together (since FX lenses don't have appropriate focal lengths on DX bodies) if I really wanted to go further with my photography. And I doubt that real photographers would want to pay good money for my DX lenses for that reason.
Which ended up in me switching to Olympus/Panasonic, who don't treat their cropped sensor size (4/3) as a second class citizen and already have a number of high quality normal/wide primes for their sensor size, but that's another story.
I guess my point is that the DX/FX issue is more an effect of who Nikon and Canon target these devices at rather than the techical merits of the sensor size. DX (indeed, any APS-C or even 4/3) sensors themselves are technically excellent, it's just that Nikon/Canon target them to a lesser-experienced audience.