[chaosdude]

For me it's a combination of many things:
WHOHOO, finally a game that does NOT rely on dumb luck with throwing dice, but actually needs some thinking.

Finally a game that looks as cool as the stories can tell it(not with simple red/blue pawns that move over an area of squares)

The models look really cool, you can play the game with every tactic you like, since there is always an army that suits your own playing style.

You can (must) paint your own models, That's the part I might like the most. Painting some grey model into a cool character.

The possibility to convert EVERY model you like. Unlike fantasy where converting gets more difficult by the restraining "regiment-based" units. That's what draws me more to 40K than Fantasy,
+ Most fantasy-players I know are of the type OH NOO! I'm gonna whine about 3mm movement. That usually doesn't happen with 40K-players.

A huge amount of freedom in what you can do to annoy/kill/fool your opponent, while the rules are still pretty clear and strict.

The fluff(which I started delving in not too long ago) really makes it a "realistic" universe.

You can play a game, win/lose it and still have fun with whatever happened.

You're not throwing some dice and count the scores afterwards, while playing and throwing dice, you can see what effects it had, and stuff happens WHILE your playing, not when the game already ended.

Someone finally came with a fun game-system for playing with armies.
We all had those people who see it for the first time and ask: Is it like chess? Or risk? NO!!!! It's actually fun to do!