Runequest took a big hit as a game system shortly before Avalon Hill folded. Avalon Hill and Chaosium/Greg Stafford had a rocky relationship for much of the time Avalon Hill had the right to produce Runequest (largely due to quality issues and frequency of the product schedule). Finally they agreed to part ways with AH keeping the right to produce a Runequest RPG (which would not resemble old RQ) and Stafford the rights to publish Glorantha material. Apparently the RQ magic system rights got lost in the mix without anyone having the rights to use those.
Avalon Hill would product
Runequest: Slayers which didn't resemble RQ much. When AH was bought by Hasbro they sold the game, and it's now available as a free download
here.
Stafford founded Issaries, Inc. and helped Robin Laws design the long promised Heroquest RPG (which was originally supposed to be high level Runequest). Instead of the tactical, detail oriented game RQ was, HQ was a storytelling flexible game. It's almost the opposite of RQ.
Hasbro eventually let the Runequest trademark expire so Greg Stafford reacquired it and licensed it to Mongoose, along with the right to publish second age Glorantha material (the "current" age is the third age).
The Mongoose RQ material has a very mixed reputation among old RQ players. It started with public criticism from the original designer of the RQ system. It was compounded by the fact that Mongoose didn't credit any of the original RQ design staff.
The early system also had some major problems in the core rules. I understand some of them were addressed in later releases. Of course, this lead to some cries of "paying for errata." Since then, I really haven't followed the RQ development and debate.
