Democracies are the only nations which can guaruntee nations of other ideologies, I believe. So Great Britain could guarantee the the Soviet Union, but Germany couldn't.
But yeah, democracies seem to be guided by the rules into being quite reactive and non-expansionist, which is why people seem to be less excited about playing them. The plus side, I suppose, is that all things proceeding relatively historically they will probably win.
Is it so easy to say that US is hands down the most powerful nation already?
US definately seems like the nation with the highest immediate potential (good industry, great resources, high manpower) but a lot of that early potential is gated off because the US starts out disarmed and with an isolationist policy. Of course, by the time the US enters the war who knows..