When introducing RQ to new groups – folk totally unfamiliar with RQ or Glorantha such as at a convention or game store – I find Pavis and the Big Rubble is probably the easiest entry point. And I also find 1625 far easier as a setting than 1620ish.
A big part of that is the relatively low amount of information needed – you have one mid-sized city (New Pavis is around 5000 people, so smaller than Arkham, MA), and a big sandbox for adventures next door. You have all the major Elder Races present, and two major human cultures (Sartarite and Praxian), with a smattering of Old Pavisites.
From there it is easy to grow the setting for the GM and players. We have an ambitious warlord with eyes on distant Dragon Pass. We have irredentist Lunar operatives stirring up trouble. Chaos cults are increasingly active. Things like the Lunar cycle, sorcery, and Lunar magic can get introduced once the GM and players have a solid handle on spirit magic and rune magic.
So what do you actually need to know? I think a little grounding on Prax and the settlement of Pavis (both Old Pavis and New) is useful, but not all that much is needed. A smattering of recent history is useful as well (this area used to be occupied by the Lunar Empire but it was recently liberated and now here you are).
More important IMO is to make sure you have a feel for New Pavis. What kind of a town is it? Where do you live? Where are your temples? Who are your leaders and allies? And also – what do you know about the Big Rubble? What is it? And what are the things everyone knows about it? That kind of thing.
