The Imperial Guard are my favourite army, the reason i love and collect them is that they have a very mortal feel about them, each grunt is a man like you or I with his own ambitions, fears and tastes, this "feel" makes the army almost unique in its fluff, sharing maybe the Tau's willingness to live, eat and breed without clouding it with insanity, insatiable greed and a heroic duty.
The Guard from a hobbyists point of view are unique as well, every Guard planet is different yet the same, populated with humans yet it could be a fascist regime, capitalist states, a democracy, a principality or even a 40K communist convergence. Each of these planets can be warm and wet, cold and dry, rocky, watery, foggy or clean thus even further shaping the development of your army. Ill pick out the Valhallans as an example, this Guard Tithe regiment is one based upon the Russians in WW2, with their iconic greatcoats and dogged "we will get it done" attitude. Their planet, Valhalla is icy and cold, thus the coats and snow bases and their frosty personalities (excuse the pun), their special weapon is the flamer, a complete contradiction of their environment.
A gamer will love the guard for this reason too, i will use Valhallans again to demonstrate. Their special character is a ruthless little bastard who sends conscripts, red raw, into the treads of tanks until they clog and stop, he does this with a special rule allowing the squad upgraded to come back on the board, signalling the original unit is dead. This tactic used in game means that objectives can be protected in the very final turn while providing body screens for more important units to hide behind.
Another army is an Airborne one. with Valkyries and Vendettas zooming around the board, missiles firing, unloading troops and staging outflank ambushes. Another? the Treadhead army, 6 oily 100 tonne tanks rolling towards the enemy, shells bursting them into bits, walkers run alongside taking pot shots at exposed heads and engines.
Yet another, the armoured assault, well trained troops with trophies and bombs taking out key objectives whilst air support keeps them covered.