This list is unfortunately growing.
Pay - if you don't know market rate, they will gouge/low ball you. Getting a raise is pulling teeth, so start high or you will have to beg every year. They don't automatically increase every year, you have to ask to get a raise.
Vacation/sick leave is all PTO. you get 15 days because Microsoft made them give vendors 15 days. To save money, they took away the 401k. no retirement now. At all. Paid holidays are only the major ones (7 or so). As of today they do not match all of Microsoft's holidays, so you will be working on MLK and President's day while MSFT is closed. (As a comparison, MSFT starts you at 3 weeks vacation and 2 weeks sick)
The Health Ins. is very good but the cost to add a spouse or kids is outrageous! Just to add a spouse it is somewhere in the neighborhood of $700 a month!! No stock (not public company), limited other perks.
You are not their primary concern, the millions of dollars they make from Microsoft is. Transitioning to MSFT is very difficult and most vendors are treated as less than human. They will try to protect you and assist if you get cut thanks to a re-org or if the contract is done early, etc. but only to help them, not you.