For Sea Org members, they will basically try to hunt you down and drag you back. Each person is free labor that keeps the machine running. They need those bodies to make the whole thing work.
For everyone else, leaving is "easy", but it comes with a complete loss of all friends and family. Scientologists are encouraged not to socialize with non-scientologists, so depending how deep you are in, it could result in losing your entire social circle, access to your family, and in some cases, loss of your job (if you work for or with other Co$ members). Even after leaving there is a fair amount