Firstly I would like to say that this game is not all that bad when compared to the other RTS games out there at the moment, however for the fans of Sup Com and Forged Alliance this game is a great dissapointment. There are a great many changes and the entire game feels rushed and less smooth than the previous two, here are a few reasons why.
The complex tech trees: Gone. You now start with one tier of unit and minimal choices.
The choices between different commander upgrades: Gone. Research will give you all the upgrades there is no choice involved.
The fine balance between extracters and power and the need to control enough deposits: Gone. Pump out those generic one tier generators and you will recieve far more mass from them than deposits.
The careful construction of defences and shields for maximum overlap protection: Gone. Spam those shields as close as possible and you can hold off the world.
The unique racial differences and strengths and weaknesses: Gone. First designer "hey guys are we going to make the units and structures have different statistics like the first game and have counter intelligence options for complex game strategy?" Design lead "nah screw that it sounds complicated, make all the shields and other structures have identical stats and get rid of the other stuff".
The detailed textures and environments that made it a great looking game: Gone. Its extremely unusual for a series to have a large gap between game and to not improve the graphical quality, this bucks the trend and goes backwards.
Well organised unit co-ordination, grouping and movement: Gone. Walking into each other, spinning on one spot trying to collect something right beneath themselves and refusing to move are now the norms.
There are more but to be honest I've got to go purge this game from my memory by playing Forged Alliance so this will have to do.