I know, I should just dive into the game, but I can't, the suspension of disbelief which is required is just too great. Or in other words I don't want to do this - I have to do this - this is just how my brain works...
I think the Imperium background is brilliant - quite apart from the buckets of flavour it gives you a moderately plausible reason to have tens of thousands of worlds, Space Marines, Imperial Guard, Renegades and various pirates, Rogue Traders and so on all at the same technical level. But then it tries to add in other intelligent species. The idea that humanity, Orks, Eldar, Slann and Tyranids could all independently evolve (and devolve) to all be at a comparible level as to have a meaningful battle is just too much. And furthermore the fluff and the rules are in conflict.
To take Slann as an example - they're meant to be highly advanced and looking on with detatched amusement at the humans who think they're the dominant force in the galaxy. The adage about not bringing a knife to a gunfight should be doubly true when bringing a bolter to a Mxcrttryrth'c fight. The idea of a stand up battle between the two species is just wrong - it should go something like this:
"OK, Marine player, deploy your forces anywhere in this area
here. Slann, deploy on that table
over there. All set? Right, Marine player, roll a d6 - that's the phase of your first turn (1 = movement, 2 = shooting, etc.) when your last character dies."
Not much of a game, is it? Instead a confrontation with the Slann should read more like a Cthulhu scenario than a battle.
On the other hand with Orks, I can accept the odd spontaneous Mekboy who takes a primitive muzzle-loading firearm and through a spark of understanding creates a matchlock, or acquires some advanced tech from another civilisation and tinkers with it such that it some how still works afterwards. They might even take on an apprentice to do the fetching and carrying. But the idea that orkish society can support the recording and sharing of knowledge, the advances in manufacturing, etc., etc., to even get out of the gunpowder age? Orks as a spacefaring civilisation? Just no.
I'm not sure where I'm trying to go with this. Just that beyond a single planet, in a sci-fi (or even science-fantasy) setting, the intelligent species should be just as "other" as the plants and non-intelligent species.
And I'm still going to paint an Astropath
