I've played Warmaster once and Epic 40K several times. My main beef is that if the normal rules (Fantasy and 40K that is) are good, they should easily convert to a large scale battle. I don't like the fact that you have to use completely new numbers and rules to play on a grand scale. The only change should be the scale of the miniatures and the scale of the stats (in one fashion or another). I think this would greatly improve the popularity of the Epic scale games - if you play 40K and the rules for Epic are the same, just on a bigger scale, you're more likely to try it out.
Interestingly enough, most historical wargames do exactly what I think GW needs to do with their games - minor adjustment for scale is the only thing to tackle whether its the kitchen table or a football field.