wow there is a lot to say about DOW
basically, for vanilla DOW eldar are OP, orc the more " skill needed race", and SM/CHAOS quite similar, more noob friendly but easely beaten by some strong eldar play
for winter assault, Imperial guard is a quit annoying race to play, it's my favorite but you need a lot of practise and skill to achieve something with, other races are definitively too strong and eldar still the best ( and it hurt to say that coz i hate eldar)
dark crusade, well there is a few changes, this one get a lot of patches improving gameplay because since beginning it was NECRON dominating everything with their too awesome resurect ability, since then it was a matter of patch increasing things for othr races and decreasing necron, so last time i played NECRON where still the best race but it was a more balanced game with other races not so far away. i think for very skilled peoples CHAOS / IG / NECRON / ELDAR where the top with little advantage for CHAOS, for more common player, skill betwen average to strong NECRON was definitively the best followed by CHAOS, IG was made really strong this time but need a lot of practise and a near perfect beginning to stand a chance to show his late game power so most people seems to hard rush to achieve victory during the ten first minuts because IG versatile commander and guard low cost made an impressive rush combo. The other improvement is the TAU race, fire warrior can shoot you before ou see them and they have a really nice big tank, more powerful than a fire prism or some nasty gorilla who can melee anything, giving bitch slap of doom all around and able to old and win ground against anything... if you can survive the first 15 minuts ... fire warrior arent good runners ^___^.
my favorite add on because IG became strong, new units introduce a lot of new strats for every races and games against overpowered necron where pure madness, annoying most of the time, but when you sometimes achieve to win you felt like a giant 
finally soulstorm, again two new races and new units, aerial this time. Dark eldars are basically quite hard to play an online they are a " spam brainlessly the winning shit, race" motojet were abused and had to be nerfed, T2 flying infantry ( forget the name) had to be nerfed and last time it was the turn of some kind of chaos beast they use who was able to eat anything, baneblade include and had to be nerfed... but like any abuse race you need to know it and play it a lot to find what and how to do 
Sisters of battle where since the beginning explained as an " imperial guard with a bolter ", well it's no far from reality, sisters can chew up basic guards but are dead meat for space marines, they can field plenty of bolters but not as good as SM ones, their anti vehicle sux hard and need a lot o play to be mastered and they have funny tanks or walkers but rarely seen online because mot peoples ( and they are not a lot ) playing them go straight for the faith power they use who are pure madnes if well mastered ( witch need time, time and time)
some new stuff, sisters have a new ressource of their own called "faith" whitch they use for their specific powers and dark eldar have one called "souls" witch they use for powers of their own
here again a lot of patches changed the gameplay more or less, last time i played the best race, not said overpowered but something near was CHAOS, followed by ORKS, ELDARS and SM. all races have a top player who can make you think this one is the best, but the one most played during tournament was ( and i think still is) CHAOS. They can tech faster than anyone, their T3 units combo is one of the best you can found, and if pushed hard their T1 / 2 is strong enough to hold ground forever ( or at least since you reached T3 )
i have less appreciated this last opus than dark crusade because it seemed that patch after patch nothing really changed, chaos was still chaos and pwned everyone and most games where mostly an eco battle to reach Tier 3 units and start fighting to win. The new aerial units where not used, exepct the new SM land speeder tornado and maybe the CHAOS bomber. Aircraft where mostly a surprise to use for huge team games than a real threat suitable for competitive gameplay use.
oups, sorry that is a looooong post