About UI:
For me as a programmer and template creator in Word, it's been a mayor annoyance the Ribbon has entered the realm of MS Office. But there's great logic in the change as well. It's not 'scrambled'. It has just been reordered in a way that is more friendly for common users. As long as you 'guess' where a common user might expect the tool or feature you need, you'll probably find it 9 out of 10 times in just 1 or 2 clicks.
About macro's:
Macro's are somewhat slower up to 1.5 to 2 times. Macro's that interact directly with your document could give a different results in 2003 and 2007. I've use seen a macro crash in 2007 that worked fine in 2003. But this is one particular macro which inserts and removes images in headers and footers. And it can be worked around. Please note that these inconveniences are in REALLY specific cases, so you probably won't ever notice.
Security has also changed. Macro's in trusted locations (ie: Startup folder) can react a bit strange to it. It's not really consistent. I've seen macro's in 2007 work, and the same macro's with the same security settings not work at different customers. So your best of here by disabling security at all, or to digitally sign your macro modules. This could be relevant when using 3rd party addins.
