I had a good post on the old Dakka called from 40k to FOW in 14 days. (the time between our 40k league and the FOW league). It got lost. I'll try to recap for you. I love both games. I really like 40k and always will. And I really like FOW. Here is what I like:
1. When you build an army list from your head to the table in FOW it is very playable. You do not have to worry about building a "Ravenwingesq" type of army that would be hard to play and you get your butt kicked all day. In FOW just pick up the codex of your choice and play. There are a lot of choices dont be afraid of German vs. US type of thing. First there are tons of codexes. Also, in each codex there are lots of choices. I.e. In US you can play Infantry, Tank Co, Armoured Infantry, Rangers, and Parachute Co. All different armies.
2. The learning curve of the game is easier than 40k IMO. In 40k one needs game time to learn how boyz with choppas do against termies, how IG fare against Swooping Hawks, etc.... In Fow it is more intuitive. One knows from life that you use your machine guns to mow down infantry and your tank guns to kill tanks. So you can get right into real battles right away without learning codex vs. codex.
3. Due to the smaller size, the board is "bigger" and thus the tactical choices are more. Also some games dont have turn limits thus different tactics. You can do real flank manourvers and other military tacitcs you read and see on T.V. That's pretty cool.
4. The imagery in ones mind is fun too, very Saving Private Ryan or the Dirty Dozen or whatever WWII cinema is in your head.
5. The small size, at first I thought I would hate it. But you get used to it really quickly. Painting is easier and yet there is a lot of detail on those small buggers that you can paint 'em like necrons or spend a lot of time picking out detail which will get noticed by your game group.
All in all I recommend it its a good other game to 40K and offers a lot.
Admin for Dakka FOW Flex League