The new Microsoft Access 2007 will continue to support user-level security (ULS) aka Workgroup Security for old-format MDB files, but the new-format ACCDB will not.

Consequently, if you have written custom code to examine workgroups and members & act accordingly, it will need to be removed and/or revised to work in a different manner.

Another item dropped from the Access 2007 feature list is the ability to use the ENVIRON() function as a default table entry; very useful items are (were):
=ENVIRON("COMPUTERNAME") and
=ENVIRON("USERNAME")
...Originally to learn these values you would need the Windows API, but recent Access versions added the Environ() as an easier method, more fool-proof than writing a declaration for a Windows API call.

I am also a bit surprised that with all the fancy ribbon buttons, the Print button is not exposed in the ordinary ribbon view - you have to click the Office button ~ Print and then choose your option: Print; Quick Print; Print Preview. Luckily the Ctrl-P shortcut still brings up the print dialog.

More differences - importing objects from Access 2003 into 2007 format yielded unexpected results - none of the code-behind in forms or reports came over. But I noticed that when I went to the VB IDE and clicked Tools ~ References there was no ref to ADO; that added OK but I was unable to add DAO as a ref "name conflicts with existing project module or library". That is the worst bad news of all, since much code has been written in DAO since it can touch not only VB but all the Access objects as well. Or at least it used to. Later, after the ADO ref was added the forms did import with the VBA code intact. Go figure.