well... i've always thought of the Imperium as a eccentric quasi-historical mix of...
"age of sail" exploration/colonization
politics of classical Roman and English Empires
honor societies of medieval knights & squires
legacy bureacracies of Byzantine/Chinese/SovBloc characteristics
but how do you "hear" the Imperium ??
http://www.youtube.com/watch?v=Kt5C9EruZ94
http://www.youtube.com/watch?v=Pdos02NooVI
http://www.youtube.com/watch?v=8gc4Yeiogl0
or...
http://www.youtube.com/watch?v=RuxHLzwlDY4
http://www.youtube.com/watch?v=FQH7s0ndQWY
http://www.youtube.com/watch?v=E-r6mU2JnKo -- http://www.youtube.com/watch?v=x9jsUaaoVJI
http://www.youtube.com/watch?v=FmZvn83G348
and seeing the Imperium as a lavish Technicolor production with current production design standards...
Age of sail exploration/colonization was at the same time as honor societies of medieval knights and squires. The Imperium's knighthood is more like nighteenth century knighthood then Medieval style. As for squires, that depends on the custom of a given planet; fosterage with a given noble that is similar to medieval squires is easily imagined.