Blue poster Daelo has good news for people aiming to solo old instances -- Blizzard has made a few changes to the way keys and unlocks work in the old dungeons, so that you no longer need keys or multiple people to get to the good stuff. The rituals in Blackrock Spire and Uldaman only require one click now, and quintessence (in Molten Core to summon Executus before the Ragnaros fight), the Scepter of Celebras (in Maraudon), and the Mallet of Zul'Farrak (in the ZF, obviously) are no longer needed to do their jobs. Additionally, Nightbane will be accessible in Karazhan without the Blackened Urn.
Some may be disappointed that items we worked so hard for are now pretty much useless, but as you probably know by now, that's the way of things when an expansion hits. The good news, obviously, is that people running through instances solo or with just a few people will be able to hit all the content they couldn't get to before. And if you happen to be running the content, you won't need to worry about whether you left the Mallet in the bank or forgot the Scepter on a Princess run.
And unfortunately, collectors will be left out as well -- most of the items will actually change form into weapons or other useless objects, so even if you have the old items, they won't be usable in the same way. Sure, the Mallet was a pain to get, but it'll be a little sad to see it disappear in the way we know it forever.