I'm not am Office "power user", but I remember using WordStar and Visicalc in DOS....
What I do not like in recent (2007 and newer) versions of MS office is that the default format for saving a document is the "X" version (.DOCX, .XLSX etc) even when the new format is not needed.
Inexperienced or simply lazy users don't take the effort to set their default to "old" .DOC, and I, as a customer or a supplier, receive the document in a format that my old Office 2003 cannot open (I have MS File format converter, obviously, but it's annoying...).
In recent years many local administrations in Italy are transitioning to open source office suites, so as a supplier or a consultant in order to present a bid or a technical expertise I must edit the .ODT document they send to me.
I've installed LO for this, and just by curiosity I opened a lot of documents I've created in MS Office 2003. No problems found. What I'll like is a replacement for MS Publisher (I used it a lot for for technical documents), capable of opening even the files I created with the 2000 version (they cannot be opened with 2003 version, and I do not have the 2000 version available anymore).
Publisher absolute incompatibility between old (2000 and previous) and new (2003 and following) versions is an example the absolute disregard of customer's rights.
When I installed Office 2003 it did not tell me : "hey, please do not uninstall Publisher 2000, or your precious documents will be lost..., and no, I do not have a converter"