I'm not an experienced guard player, perse, but some tactics I have found or thought of to be beneficial are:
Always remember, tanks are your strongest unit pretty much. Nekkid
LRBT's(150pts ea) are stronger than you think, and are arguably the best tank for it's price. Also keep in mind the Exterminator and Demolisher
Medusa's with BB shells will more than likely to do damage to a
SM Landraider (S10, AP1) so keep that in mind.
But pretty much, all of the new tanks have their own plus-side. Also, keep the Artillery Sergeant Pask in mind for one of your
LR's. improves from bs3 to bs4, I think you can reroll wounds(?) and you scatter less, which is a major plus
lol
Power in Numbers are just about as true for the
IG as they are for any army. 50 standard guard = 1
KP and roughly 250pts clean, as opposed to the Marines, where my
GK Marine squad of 10 costs roughly around, if not over 300pts, for 1
KP. So do the math there
Creed and Kell are very beneficial. I don't know if they still require a 1500pt list to be used, but 4 orders and increased range can go a long way.
Some orders to keep in mind:
-Bring it Down!
-Fire on my Target
-First Rank, FIRE! Second Rank, FIRE!
Also, Commissars are (rumored to be) a good investment, especially Lord Commissar (cuz what guardsman doesn't love getting shot in the back of the head with a BS5 Bolt Pistol?)
PSYKER BATTLE SQUADS!! 10 per squad, at the least! Anything less than 10 per squad and you're wasting your time
lol
Hmmmm...
Vet's are considered troops now, BS4, basically an all-around higher statline than standard guard. but you're limited to only 10 per troop choice, which isn't bad. it's actually better if you enjoy a speedier deployment. Use them with Chimeras and always have Special weapons. Meltas are generally mandatory among vets. There's also Sgt. Harker, who gives a vet squad some good bonuses.
Vets are also used alot as Air Cav, since Meltas aren't meant to be walked across the field
50 man blobs of standard guard are delicious for you. I recommend having Vox's (re-rollable
Ld tests for orders) and I forget which character gives it, but you can give them Ld10 pretty much, so ya.
FRFSRF x 50 = 100 @ 24", 150 @ 12" and if anything gets too close to you(within assault) and you're feeling like charging out with 100 melee attacks... yah, 'nuff said.
You can have tank squadrons, of 3 tanks per squad. (so 9 tanks fill up all Heavy Support slots) but they can end up being huge point sinks. I recommend only have 2x2, or 2x3 (3 squads of 2) and your tanks must maintain coherency and must fire at the same target(per squad)
in terms of going against
SM, aside from the Medusa, invest in a lot of Lascannons and Meltas, and of course a Psyker Battle Squad. There's a new melta-cannon, with 72" range, but at BS3 w/out Pask.
in 50 man blobs, you have 5 Lascannons per merged-blob. so ya

Psykers will put out a pie plate with strength equal to their squad size (10x psykers = S10 with an
AP based on
d6) so you can get lucky with another S10 AP1 pie plate, with 36" range I think they have. so ya.
Oh ya, and also, it seems to also be mandatory to have two Company Command Squads. able to give out even more orders, and I forget the strategy for them. I'm sure someone will post here mentioning it.
If you gave a point limit for a battle you're thinking of, I'm sure everyone here could help you out a lil more with making a list.