In my life experience in the nearly 5 decades since the early 1960s the following have perpetually only been 20 - 30 years and just a few tens of millions (or billions) of dollars away...

+ Cure for cancer

+ Cure for common cold

+ Manned mission to Mars

+ Fusion power plants

+ AGI Artificial General Intelligence

+ Household utility robots

Flat screen wall picture TVs finally got tossed out of club membership this decade (the new thin LED TVs not the fat 5" thick LCDs)

+ Hydrogen or fuel cell cars (OK, more recent entries to the club)

+ Cheap ubiquitous solar power panels etc. (I think this is next to be tossed out)

Any other nominations?