Excluding the over the top crap I hated the game because:
1. Resource management is destroyed. In old games you had to protect your harvesters and scout out new ore fields before decding if it was worth the effort to use resources to capture them. In this game you just build a refinery right next to the ore depot which removes the entire strategy of wiping out all the miners so the enemy base stagnates.
2. The water. EA said the water would be as important as the land; instead I get level after level of islands, bays, beaches and rivers. It was nice at first but water is just water while in Red Alert 2 you had snow fields, cities, grass lands, deserts, gorges and rural communities to wage war around. Basically it felt like the water was a substitute for something much more interesting that could have been there instead.
3. The second abilities. In Red Alert 2 only a few units in each faction had second abilities, it made them unique and a valuable asset. Now everything has an ability so everything feels average.
4. The conversion abilities. Watching the trailers I thought it would be awesome how some boats could go on land and vica versa, but instead they all do it. Since most boats have cannons and the enemy base is always on the other side of water, it just felt like unnecesary micromanagement to build tanks and transports rather than swarm with boats. This made all the land vehicles feel useless and made the game feel like I had far fewer units to play with.
5. The secondary AI. Great at first but I couldn't turn him off. After the first level or so it felt like I was just having my hand held for me, this became particularly apparent when while I was building units for my next strategy, I see that he has already swarmed with boats and the enemy is dead. The final straw was when I weakened an enemy before moving in with engineers for the classic C&C base steal. I get my engineers to the island only to see his bombers fly over and blow up the last of the buildings. Granted I could tell him to stop but then I am working against the AI not with it.