Modern Units? I think it's the relevance / reality / current possibility of it, as others have stated, that drives & keeps my interest. I've just started playing with CM, it's been a long while since I did a WWII game, and I keep being struck by what a difference there is in an infantry squad these days, firepower-wise.
And of course, that doesn't even take us to heavier weapons...
I'm having fun w/ CM, but I bet one of the biggest effects will be to drive me back to more TacOps (been playing off and on since '94) -- well, that and my v.4 should be here soon!
I find myself pondering the differences - fundamental principles are of course the same. I think the adjustment for many WWII gamers will be a change of scale - much longer engagement ranges, terrain allowing. Of course, most game map scales are adjusted to reflect that. And, as mentioned, much less "second chance" when you are engaged.
They do exist, though, the second chances, and I've sure seen those stick in your head amazing feats that make you want to award medals to game markers in TacOps. If you haven't given a 'moderns' game a whirl, I sure can't reccomend one more highly.