Other than their political affiliation, they all received discounts on their mortgages.
From The Los Angeles Times:
Shortly after joining the U.S. Senate and while enjoying a surge in income, Barack Obama bought a $1.65-million restored Georgian mansion in an upscale Chicago neighborhood. He secured a $1.32-million mortgage from Northern Trust in Illinois.
The freshman Democratic senator received a discount. He locked in an interest rate of 5.625% on the 30-year fixed-rate mortgage at a time when such loans in Chicago averaged as much as 6%. The loan was unusually large, known in banker lingo as a "super super jumbo." Obama paid no origination fee or discount points, as some consumers did to reduce their interest rates.
So, is this a standard perk for senators? Bet you won't hear much about it in the old 527 media.