My biggest problem with WarPlan is that its schizophrenic: Is it meant to be a light wargame, or a realistic simulation of World War II?
Some features are very gamey, notably the Axis & Allies-esque capturing of enemy flak guns, and the easy amphibious invasions under the noses of the enemy's navy and air force. Add in the lack of stacking, and you have a Strategic Command-like game rather a deep historical simulation.
There's nothing wrong with that, except that some features of the game resemble the fiddly mechanics found in more complex wargames. In particular, the logistics rules such as tracking how many units can be supplied by a port. Or, the need to constantly look over your forces to determine which units have winter or anti-tank bonuses, or whether to build your armor with a breakthrough vs heavy armor focus.
There's nothing wrong with those features, either. Except that WarPlan becomes a sometimes awkward juxtaposition of light wargame and complex simulation. The intent may have been to create a game more realistic than Axis & Allies but less complex than World in Flames. To that extent, I think Warplan succeeds. But as the design stands now, I think it suffers from a dual identity.