IR: The IR faction identity is to beat anything in its way down to pulp. In space this usually entails SSDs as a centerpiece for a mostly capital/heavy frigate fleet. They have terrible fighters and bombers that are usually deployed en masse to overwhelm the enemy. On ground the Empire is very much the "slow and steady" faction. A favorite tactic is to bring in two AT-ATs, a SPMA-T, two units of AT-STs, and the rest NovaTroopers. Slowly advancing towards the enemy base, relying on an impenitrable wall of forward facing firepower, that is Imperial ground for you.
PA: Really the only other faction I've done extensive play with, the PA faction identity seems to be that of adapting to whatever forces you face. Early PA space will be practically solo combat with your SSD and very little support. Later PA space will be a mix of classic IR strategies, or more NR based fighter strategies. Either way you go, though, most likely your offensive fleets will contain 1 Praetor as a damage soak/flagship. A typical base fleet for my PA space would be: 1 Praetor, 2 ISDs, 2 VSDs, 2 Venators. From there I will add or replace ships depending on what I am fighting. If its the IR more damage dealers, the NR more corvettes/carriers, the EotH a little of each. For PA ground, they are much faster than the IR; though with the capability of similar strategies, I use them much more as a fast attack fleet than a wall of death. The units I use the most are: Floating Fortresses, TIE Crawlers, LAAT/i's, Hailfire Droids, Pentastar Enforcers, and Storm Commandos (for quick RP capture). They are faster and more flexible than the IR ground, as each PA unit can function in multiple roles.