I've never played/read it but isn't it just D&D without the name?
That depends highly on one's definition of D&D. It's a close derivative of D&D 3.5, but at the same time, it works differently in character generation, especially skills, so, it's clearly NOT the same game, tho' it uses the same core engine.
Kind of like comparing T:TNE to T2K2.2 - yeah, they're clearly related, closely even, but the experience is different enough that not everyone agrees they're the same, especially since CharGen is different.
Star Wars I would not really classify at science fiction at all (though I am a fan of the movies). I'm not familiar Warhammer 40K but the impression I get is it's just orcs and elves with ray guns and space ships.Star Trek, though very patchy, does often feature good science fiction elements.
Star Wars and WH40K are both Fantasy in Space. The difference is one of Tone more than of realism levels. Both have non-newtonian space movement, both have nearly universal biocompatible life. Both have stupidly large ships with crews into the half a million range (tho' the lower ends differ a lot), both have major use of psionics.
Essentially, the 40K setting can be seen as Traveller crossed with Chtulhu, with Fantasy races used. (Mind you, Traveller's gone Fantasy Races at various points, too.) It's worth noting that 40K comes out right after GDW cancelled GW's Traveller license... and they'd been working on an ATU. The factions in the 40K universe being:
- Humans - essentially an evil empire and its army and space marines
- Eldar (space elves),
- Orks & Gretchins (Orcs and Goblins)
- Tyranids (who start of with the Geanstealers - a dangerously close parallel to the Alien series' antagonists)
- Necrons (ancient's Terminators or Borg)
- Tau (proud warrior race)
- Chaos (chtulhoid monsters and the mutated humans who worship them)
I enjoy the hell out of reading the 40K setting fluff. The early stuff could easily be done with CT; the newer stuff, well, it's gone a bit afield. The tech paradigm is no longer Traveller compatible.