Sorry, late to the party here.
Going back to Smiler’s original question, I agree that new players can only be a good thing, although I’m not sure I share his focus on Oldhammer rulesets, I’d prefer to focus on the ethos. Although I will contradict myself further down this post…
I think the most likely source of new players is existing wargamers, who I think it’s fair to say would have some exposure to the current or at least some version of WFB / 40K.
In that context the biggest selling point would be the DIY attitude. At its most basic level this releases the player from the cycle of forced obsolescence / the whims of the sales and marketing activities of a miniature manufacturer. Hopefully it also gives priority to playing a game rather than focussing on winning, on seeing the units and heroes as characters rather than pieces on a board. Certainly when I've played recently at my local club the people I've chatted to have seen how 2nd / 3rd ed let you do the first of these and seem to like the idea.
On Smiler’s specific questions –
Why learn an old rule set – I don’t think there’s any particular reason why anyone should. I think they should use a complete ruleset, which RT / WFB 2nd (and sort of 3rd) edition are, otherwise a lot of the freedom to make up your own stuff is lost. So it should have a full selection of creature profiles, plus a framework to make up and cost your own, and a representative set of spells and magic items. In my personal view it should also allow for a wide disparity in power levels – so that your chaos warrior really has very little to fear from a goblin. Although I note that this last thing is specifically something that the designers of my 1991 edition of HOTT were trying to counter.
The obstacle to old rulesets is primarily that they’re out of print so you’re creating a barrier to entry right there, which is why I don’t think this should necessarily be the focus. Although contrary to that is it’s nice to have a lingua franca - it’s also a barrier when you find a fellow gamer who shares your gaming ethos but you don’t have a ruleset in common. Of these two problems the ruleset is the easiest to overcome though.
To slightly twist the question to me the obstacle is the last 20+ years within fantasy / sci-fi wargaming of focus on competitive play and proscribed forces rather than just making stuff up. So run scenarios where one side is clearly unmatched (Orc’s Drift’s Encounter at Ashak Rise is a fantastic example of this), or just where players have to make do with the forces at hand within the scenario rather than a finely-tuned picked army. And I just love Gaj’s suggestion of generating non-typical characters via WFRP.
As far as encouraging this sort of thing as a community, I think in many ways we’re doing a reasonable job already although I imagine the focus on old lead is somewhat off-putting. Play using cheap and/or current figures - a la Whiskey Priest; play using modern editions; enjoy yourself. Something which I imagine would be useful – although might be fraught IP-wise – would be some guides to DIY-ing modern editions, or Oldhammering a non-GW modern ruleset.
Less likely to get you sued would be to focus on smaller forces, again lowering the barrier to entry and allowing for more variety. I think it’s instructive that the scenarios I grew up with (Vengeance of the Lichemaster, Blood on the Snow, Forenrond's Last Stand) have sides in the 1000-1500 points range which have lots of advantages (as a player rather than manufacturer) over the 3000 point sides that Warhammer Armies made standard.
So, be creative, enjoy yourself, and I'm sure you'll find a steady trickle of disillusioned former GW-gamers coming your way. It will always be a niche of a niche though...
Paul / Grumdril