Actually, it depends on the game, but quite a bit can be left out of the
"warez" version of the game.
First, by necessity the "warez" version has to have any copy protection or CD
checking removed. Any files which would have loaded from the CD have to be
repathed to load from the hard drive. It's not overly technical, but any
errors introduced in cracking the protection can have an impact on the overall
stability of the game and how well it plays.
Second, because of space and bandwidth considerations, anything deemed as
extraneous has to be removed and the warez version has to be segmented to make
it friendlier for downloading.
Typically any movies & cut scenes, help files etc. will be removed from the
warez version. While removing a movie doesn't affect the gameplay, once again,
errors introduced in this process can also affect the way the game plays.
Finally, a warez version that hits prior to the release of the final game might
not be the final code, and it may not be completely debugged or optimized.
Testing versions and pre-release versions that are sent to reviewers sometimes
leak, and these pre-release version find thier way onto the web.
So, if people complain about slow games, crashining or other errors prior to a
games release, it is a valid question to ask which version of the game they are
running, and how it was obtained.