I was quite an avid defender of the game after the first screenshots were shown, and only really became critical when we knew the game was coming with one major game mode - Before release, this was my big peeve. Even DoW2 could have benefitted from more fun game modes than the two it was given (Three including the brilliant TLS).
In retrospect, it was the sheer lack of races. Buying Dawn of War and having 4 major races was brilliant, and all 4 were fresh and new because it was the first of the series. Every addition was a straight buy because of how well the first 4 were handled. Dow2 introduced my favourite faction and came with 3 of the 4 races from the previous game, which I was fine with as Tyranids threw a big new gameplay mix in, and we eventually received Chaos and IG back and they both played in a very fun and different way to DoW1.
DoW3 having just 3 races, and the 3 safest races in the entire 40k universe, was a huge disappointment. Many players have paid a LOT of money over time on the series and when each iteration requires even the thought of having to buy our favourite races AGAIN, it puts people off before they are even interested. On top of that the races were fairly similar in play, with the elites taking the spotlight and the regular soldier becoming somewhat of a grinder fodder unit.
I feel like just introducing one new race, or an old favourite, would have been a huge boon to the games release. Space Marines, Orks and Eldar are done to death, and each are fairly similar to the fantasy races they're based on - Obviously anyone who is a big 40k buff knows how great these races are, but to the outside eye, races such as Chaos, Necrons, Tyranids and Tau have more of an edge. Imperial Guard are one of the most popular races in 40k because of their human vs insanely powerful aliens and bring in the insane cavalry theme.
There's a lot that could have been done "better", but the same could be said with DoW2 as well, there are reasons people stuck with it and it succeeded. The thought of buying and unlocking elites didn't have the same excitement to it as using whole other races. A lesson that CA seems to have nailed with TWW and its wide supply of unique races.