Anyways, I suppose I can't speak to others' use of the term but at least when I say a "beer'n'pretzels" game I don't mean so much to do with the actual game mechanics themselves as the incoming attitudes of the players. If you're sitting down for a low-key, casual gaming session that to me is a "beer'n'pretzels" game and in that type of setting I find players are less likely to worry about balance issues, to a point. Of course massively uneven sides are still no fun (usually for either player, although obviously less fun for the disadvantaged player), but if no one came to the table with a WAAC list and instead both built what they thought were just fun and fluffy armies, I think there's less of a chance that one is going to a) stumble upon a killer army, b) play that army terribly efficiently, and/or c) be as hung up on how well the army does.
Obviously, as has been pointed out in this thread, some fluffy armies in some games are more competitive than others, but that doesn't mean that the player has necessarily realized it or that the player is going to use highly competitive strategies. Depends on the players and the atmosphere.
So it's not that a fun game is an unbalanced one, but rather that an unbalanced game can still be fun if it's not a particularly competitive game, again to a point. Huge discrepancies in balance would of course still throw that off.
I have no experience with WM/H, but as I understand it that's not really a game where you build your list based on which models you like the look of, and if you're not playing competitively you're probably not going to have a lot of fun because you'll get hosed every time. In a game like that, balance is a lot more important, because it's the balance that allows highly competitive play to be fun. And of course that's the issue with GW games - there are a lot of balance issues and at the same time a lot of people do enjoy playing them competitively.
I'm not saying that beer'n'pretzels games can't be balanced, but rather that noticeably unbalanced games are almost doomed to only be beer'n'pretzels games, since you can't be too serious about winning or losing and still have fun. And as before, some semblance of balance is still important. The magnitude by which one force is just plain better than the other has a lot to do with how well you can just sit down and enjoy a game. If one army has a clear, but small, advantage that's not such a big deal.
Funny you should bring Mordheim up because, while a lot of fun (and my favorite GW game by far), it is definitely not what I would call a balanced game. I do agree, though, that with the amount of stuff going on with any one figure in a game of WFB/40K they would be much better suited to 20-30 models per side.Now, admittedly the problems with 40k and WFB stem from their, shall we say, obsolete nature? The basic mechanics are rooted in gameplay that works on a small scale (20-30 models at most on a side), a statement amply proved by the playability of Necromunda/Mordheim. 28mm minis are just not MEANT for large-scale games.
Also for what it's worth I'm definitely not opposed to competitive gaming or playing to win. I think my post kind of reads that way, which isn't the case. Just where my opinion falls on balance issues - balance is crucial to the enjoyment of a more competitive game and without finely tuned balance a game is better off in the "beer'n'pretzels" category because it can still be enjoyable. Just depends on what you want out of that gaming session.