I think the hardest part of vetran is figuring out when the enemy respawns, when the allies will help, and where the pseudo-checkpoints are (ie where to advance to so the allies will follow and then the enemy stops re-spawning).
The reason I found CoD3 harder than CoD2 was not because the game was actually harder, but because I could not re-do the level on easy, figure out the above issues and then play vetran. Similarly if I was bored with a level in CoD2, I could skip around and do something else and then come back. Its a shame, but I think the game would have been spectacular if they had coop for the campaign. hopefully CoD4 will have that.