I play IG with a more or less pure infentry army. I have found that it is a very potent force.
The first thing, and probably your best weapon is that most players will over look all of your cruddy little men with their glow wands.
But glow wands enmass are a strong weapon. I once took out a very sizable unit of genestealers with the combined fire of about three squads.
I themed my army on candians and stuck with the fluff so have every squad with a autocannon and grenade launcher. it is a very good combo i find. I can munch my way through most enemy infantry, and have enough numbers to not have to worry about enemy tanks.
I also use a vetran squad with shotguns and three grenade launchers. their extra BS and all their weapons being assault along with flank attacks make them a VERY good unit, well worth their money. they once had fun against a necron destroyer lord, killed that thing FOUR times!!!
Their is still the option for tanks if you want them in a normall IG army. three leman russ's if you so wish. though i would take two and an basalisk if i where you.
the new IG book is ment to be having a lot of the old tanks re-introduced as well as the odd new one i think. And what with SQUADS of tanks, their is going to be a lot more armour rolling in gaurd armys.
saying that, a LR with sponsons and a few upgrades is about the same as a basic guard platoon with heavy and assault weapons. except for the battle cannon, the platoon has much better weaponry and will last longer.
wow... you can tell i aint been on for a while