BQ - I think the 00's were fine overall but not if you take into account what was popular or mainstream, going by that the 00's were a complete disaster and by the mainstream music has been since post grunge/boy bands/nu metal became popular back around 97 or so.
BQ2 - that will be tough to gauge I think the 70s and 90s have it beat, 80s it may be comparable to though and 60s i don't really like besides the late 60s. based on mainstream music it is by far the worst decade out of those though.
BQ3 - this is always tough
Immolation - Unholy Cult
Opeth - Blackwater Park
Enslaved - Isa
Arcturus - The Sham Mirrors
Emperor - Prometheus: The Discipline of Fire & Demise
the 2000s were pretty great for metal, rock they were absolutely terrible for.
BQ4- hmm it wont be remembered too favorably I don't think, the mainstream music will be remembered as being very weak but people will probably think of emo/posthardcore/metalcore, pop-punk, post grunge type modern rock(nickelback, papa roach and all that) nu metal and rap the use of autotune, disney pop groups etc.. not really great things to be remembered for. music fans will remember the great underground releases.
BQ5 - I hope the next decade the mainstream gets a lot better and the underground remains awesome.