I suspect that people don't like something that's unique. It's the same reason I bought my Z4M for $18,000 under sticker brand new.
It's definitely not slow though. I'm ok with a high 13 second quarter mile. Then again there are PnP chips that take it to 550 lbft at the wheels. But I shall be good in the name of the warranty.
The 335d isn't really that unique like a Z4 M because a Z4 M performed better than normal Z4s. The 335d just sort of did with gas but it cost a lot. when new, it was well more expensive than a 335i and only slightly faster than a 328i e90 (and im talking stock here). plus it was heavier than both so it didn't handle as well. Then the enthusiasts didn't like it becuase it didnt come in manual, and obviously people who wanted to save money on gas probably did the math and figured it'd take forever to save enough gas to make up for the fact it cost more than the 335i.
I remember when the guy i knew bought one (and at the time I told him it was a dumb idea still) it was like $4500 eco diesel credit and another $5000 off sticker because it was april 2012 and it was a 2011 model (and more or less rotting on the lot, i think he could have gotten another $5000 off if he had known how to haggle)
As for the Z4 I never got why the e85 didn't sell so well. some people didnt like the look I guess, but i don't think you could say the E85 was ugly. the e89 Z4 I think is gorgeous, too bad they didn't make an M version.
The 320d will be a nice experiment. I mean lets say it gets 35mpg combined. so people who have long commutes might want it, but it has 180hp and 280 torque so itll be the slowest F30. I'd imagine they won't be able to price it much more than say $2-3k more than an equivalent 328i, but it might just work.