Changes in the Max Environment

Preferences
If you are a user of an older version of Max, your preferences will not be imported to Max 5. Preferences include the search path additions you specified in the File Preferences window, as well as DSP settings and MIDI port mappings. You will need to recreate these settings. Saving Max 5 preferences will not overwrite those of older versions.
The Search Path
A Mac-specific feature some users may have used in the past was the ability to alias a folder within the search path. This is no longer supported in Max 5 unless the alias is specifically added to the search path. For example, if you have an aliased folder called MyWork, you cannot drag it into your patches folder and have all its contents available in the search path - you'll need to add it manually.

Adding an aliased folder to the search path

File Extensions
Max 5 is more sensitive about file extensions and uses them to determine the type of file you are trying to open. For example, when saving a Max patcher file, you must use the .maxpat extension or your file will not open properly.
Some Macintosh users of previous versions of Max may have been somewhat casual about file extensions, instead relying on the Macintosh OS's legacy "file type" mechanism to identify their files. This is no longer advisable. Furthermore, when importing old documents, Max 5 pays more attention to the file extension that than it does to any Macintosh OS "file type" information that might be associated with the file. For example, if you have a text file and you have made the mistake of giving it the extension .mxb, Max 5 will think it is a Max binary file until you change the extension.
This has always been the case on Windows; now it is the case on the Macintosh as well.
More information about converting old patcher files.