What are the big differences between the Design Mechanism's RQ6 and RuneQuest 2nd? I mean, if someone, hypothetically speaking of course, owned RQ6, why would they consider RQ2?
I'm not sure you would get much out of the RQ2 ruleset if you have come to RuneQuest for the first time from RQ6, apart from curiosity and old typesetting, maybe some old typos? The RQ2 rules didn't have that much Glorantha setting beyond the magic system and the creatures, the setting came largely in the supplements. Personally I wouldn't want to go back to RQ2 for the rules because I don't know why I would want to 'forget' the new parts of RQ6 that I like.
There's an association with new rules with bloat that I don't think is true with RQ6, you can see it in RQ3 which had parts which didn't work very well like sorcery, fatigue, cultural backgrounds, all of which were addressed in RQ6. RQ2 has some oddities which I'm sure some people like but which seem like munkin-fodder to me - escalating Battle Magic, Blade Venom and slugathon-combat, which again RQ6 handles much better, especially with special effects. The idea that an older ruleset is by it's nature streamlined and therefore faster and easier to play doesn't pan out, combat in the new RQ is more interesting and plays out faster.
A nostalgia reprint is a nice idea but it appeals to the record collector in me not the player.