6mm is definitely better for Epic than 10mm or 15mm. Epic is all about maneouvre warfare, and maneouvring requires
room, which is why a 6'x4' board is a
minimum size tabletop, to my way of thinking.
I think the problem Epic faces in finding a significant market, above and beyond the "once bitten, twice shy" attitude GW exhibits, is it's complexity--which, ironically, is the very thing that makes it so awesome.
Epic is conceptually more complex than 40K due to it's higher level of abstraction. In my experience, young kids (under 15) who play 40K tend not to get Epic, as evidenced by the "Why do Devastators only carry missile launchers?" question I keep getting.
I think it would be in GW's interests to treat 40K as a "gateway drug", and seek to retain aging 40K players (the 20- and 30-somethings) tired of 40K's limitations by transitioning them into its
real wargame system, Epic.
If I was running GW, that would be my strategy: a player bored with 40K (as I eventually became) all too easily leaves the fold entirely, and then ceases to be a source of revenue for GW; whereas a player bored of 40K who is then
introduced to the larger world of Epic is a potential on-going source of revenue, and one that was cheaply won (in marketing budget terms).
Just my 2 cents, as ever.
