I just want to relay a sad experience from last week. You see, I set out to play at a new club in a city nearby and ended up playing a 2000 pt game against Chaos. In total, my list looked something like this.
HQ w/creed and 2 bodyguards
1st plt HQ with plas/melta/vox
3 Inf Squads (GL/Plas/Plas)
2nd Plt HQ with Plas/vox
4 Inf Squad (Plas/GL/GL/GL)
x2 ML Sqds
3 sentinals (individual) 2 w/LC
I'm not sure on his list. But I know he had a Vindicator, and LC Predator, 4 squads mounted in Rhinos, and a HQ Squad mounted in a Rhino. I stole the initiative and got to go first, moved most of my Inf up 6" and promptly destroyed a Rhino and the Vindicator. In his turn he moved the now dismounted squad to cover, and moved the other 4 rhinos up and popped smoke. Now is where things started sucking!!!
I dicked up my turn by giving my orders to my 2 ML squads, and then (here's where it got bad) instead of shooting with my tanks/arty I just continued on shooting with my Inf. You see I had just shot with 2 ML squads and just seemed to go with the flow and stay with infantry squads, BIG MISTAKE! In total 3 of my 6 Plasma gunners killed themselves, and not one did any damage. So, now it's time to shoot the big guns, and of course they knock out NOTHING!! Yup, he made every one of his cover saves due to smoke....
See where this is going? At this point he dismounts, moves six, and assaults six, and half my army is now engaged in CC... and we all know how good IG does in CC.
Moral of the story. Pop the damn transports with your armor and heavy weapons BEFORE shooting with your infantry squads. This is common sense and every commander should know this, but in the heat of the battle you need to keep a clear head and stick to the plan... I'm sure there's other points to be made as well, but I really think this was the biggest problem I had.