The current release of the Steam client has a bug which is preventing some games to fail to launch, including Bastion, Bleed, DLC Quest, and SpaceChem.
EDIT: Unfortunately, issues are still ongoing with GTA3 and GTA: San Andreas. We are aware of the issues, have a good idea what's going on, and are working with Rockstar to resolve the issues with these games.
EDIT2: GTA3 and GTA: San Andreas have been fixed now.
We have pushed a fix for this issue to the Steam client beta, and we are hoping to release it to everyone soon.
To opt into the Steam beta, select Steam -> Preferences… from the Mac OS X menu bar. Click on the “Change…” button in the “Beta participation” section, and then select “Steam Beta Update” from the dropdown. Once you’ve clicked “OK” to accept the change, Steam will download and apply the beta.