I've posted my thoughts on Access 2007 and I'm just pasting them here for convenience:
1) You can get your toolbars working again with a few setting changes:
http://accessjunkie.com/faq_31.aspx
However, you can no longer modify them graphically! You were able to just drag items or create new menus and so on with a few clicks that now need to be modified either by code or 3rd party tools. If you have 6 or 7 menu items with 6-10 selections each, be prepared for a lot of difficulty.
2) Exporting reports to Excel. Whoops...Microsoft removed this ability. Their work-around is to "just" create the corresponding code to export the table or query. That's fine if your database has 2 or 3 reports but if you have 40 reports and you want to be able to export any of them as needed, you're out of luck. Of course the nice grouping and totalling is gone if you do this as well unless you're into writing piles of code to do something you USED to be able to do with one click.
3) Filters. One thing that seems to have changed is how you refer to your combo boxes or whatever provides the filter data. I haven't seen this documented anywhere but I found it to be a problem.
For example:
In the past if you had a combo box (cboLastName) that you drop down to see everyone in your table whose name is Charlie:
Me.Form.Filter = "firstname= forms!frmMyForm!cboFirstName"
After people getting Access 2007, I found that this stopped working. Must have made a lot of people smile that the program they've been using for 8 years stopped working.
What I found you have to do now is use quotes:
Me.Form.Filter = "Firstname = """ & forms!frmMyForm!cboFirstName & """"
Best of luck on the retro-fit.
4) New useful feature: the properties window for forms and reports can just disappear and be impossible to open. I have found out that this happens when you disable some toolbars using code. You need some more code to make the properties box open. I'm not sure why disabling the toolbars removes your property window. Whatever the rationale is, I guess Microsoft didn't think it should work that way for the past 15 years of Access.
6) Another surprise: if your users like to navigate using the mouse wheel as mine do, make sure they are prepared to lose that ability. Unless you use some code like Allen Browne's.
5) Last really cool thing: the time that it takes to refresh screens. I have one popup screen that "rolls" open and closed using just a few lines of code. It used to look pretty cool and take approximately 1.5 seconds for the animation. The same screen in Access 2007 - let's just say you can get a coffee and cigarette by the time it's finished. This is even on new computers with lots of RAM and good video cards.
There may be other things to be aware of but these are so far the ones I encountered.
SHADOW