Soviets: You can only build half the units in your army at best, which except 2 (maxim & sniper) all of them are shit compared with axis counterparts. Stupid vet bonuses that doesn't help at all. No infantry play at all except shocks.
USF: All starts involve spamming rifles. Stressful gameplay by knowing that you must win the game in 20 minutes or your lack of heavy tanks and proper AT units will make the lategame a impossible battle to win.
Ostheer: Fun and several strategies to choose from the beginning. Non-doctrinal units are powerful. Although T4 units are good, they don't be used much because doctrinal ones are better.
OKW: Fun gameplay that involves gambling and taking risks placing your trucks. Best non-doctrinal units in the game. Crazy vet bonuses that reward good micro. Sadly, encourages blobbing infantry.
And still people wonders why nobody wants to play allies?? 
complete bs
soviets have shocks, which are the best anti infantry infantry in the game. dont even try to say obers are, they do not have the durability that shocks have. t34/85, is2 are awesome. with isu retarded monkeys could win.
USF win in 4 minutes with rifle spam no stress at all. spam e8's and jacksons no stress. crazy good double lmg rifle squads followed by jasksons.
ostheer restricted to grens stug tiger. so much fun its unreal.
okw-espensive as fuck everything but shreck volks. no resource. no fun. up against maxims? absolutely no fun.