At the level of language compatibility, FPC is very compatible with Delphi: it can compile most of FreeCLX, the free Widget library that was shipped with Delphi 6, Delphi 7 and Kylix.
Currently, the only missing language constructs are:
There are some inline assembler constructs which are not supported, and since Free Pascal is designed to be platform independent, it is quite unlikely that these constructs will be supported in the future.
Note that the -Mobjfpc mode switch is to a large degree Delphi compatible, but is more strict than Delphi. The most notable differences are: