Well, back to the original question BI is just that - Barbarian Invasion.

Almost every barbarian faction in BI requires you to have your guys expand into Roman territory and knock out one of their regions, while avoiding any other barbarians who want that same territory. It's a kind of revenge against the Romans (Julii, especially) for conquering so much land 450 years ago.

Obviously if you're the Romans, this is where it gets interesting. The Eastern Empire doesn't need to worry about money and religion (there are official religions and if the majority of people are a different religion there will be serious Public Order problems), but have the potential of massive armies from the Huns, Vandals, Sarmatians, Goths, Roxolani, and the Sassanids just waiting to destroy you. With the Western Empire you have to deal with all that plus serious religion, money, and loyalty problems.

As a final point, the PBMs that econ21 advertised are a great read - the WRE one inspired me to get BI.