Flat Organization - Very little room for growth. No "ladder" to climb. Of the 1400+ employees, I would guess 1000 or so are Implementation Consultants.
Basic Technical Experience - As an Implementation Consultant, you basically use FAST's COTS (Commercial Off-The-Shelf) software, and limited T-SQL and VB.NET. You are not developing software unless you work at HQ in Denver, and you are just configuring the pre-developed software to fit the needs of the specific government entity you are working at. If you want to get software development experience to grow to potentially move jobs in 2-5 years, this is probably not the job for you.
Relocation - Not a con to all, but only being at a site for 1-4 years before being required to move means most people don't buy homes, meaning they rent for the entirety of their career at FAST, and they don't get to live where they typically desire.
401k - While profit sharing exists, I would much rather see FAST drop profit sharing and add 401k matching. I would rather benefit greater from my contributions to my 401k than benefit less with profit sharing.