True. Bioware screwed over the Infiltrator compared to the Soldier in ME2, when they made the Cloak Damage Bonus additive, while making the Adrenalin rush one multiplicative, making the Soldier the more powerful Sniper. Fortunately in ME3 Infiltrators are the better again.
I haven't seen the statistics for ME3, but I know Bioware released statistics after ME2 that showed the large number of players using the Soldier class for ME1 and the majority of players using the Soldier class for ME2. Bioware also said how many players switched their characters to Soldier when they transferred their saves from ME1 to ME2. My first ME1 run was an Infiltrator, but I felt the game pushed me to play like soldier. So I followed the herd and played as a Soldier from then on.
It was the guns. Only the soldiers got all the guns. Vanguard is probably more fun (but nobody in MP wants to play with a vanguard) and sentinal/engineer are supposedly more difficult(wouldn't know ... not enough guns)
Guns and heavy weapons. In other words, guns, bigger guns, rockets, and staffs of fire, frost, lightning, banishment, and annihilation. Wait! I meant Firestorm, Avalanche, Arc Projector, Blackstorm, and Cain.