Though Microsoft claims that their Office suite is very "interoperable" with other sets of software, we all know that the truth is far different. It was those differences that upset the UK Government, who complained to the EC demanding that Microsoft make their software more compatible with other products. After waiting for a response, a British agency has now filed another complaint against Microsoft with the EC, claiming that Office 2007 in particular works to impede interaction between it and competing software. They also complained that the licensing practices employed by MS for schools are anti-competitive.
There's no real surprise in the complaints, as everyone who has tried to use other Office suites has probably run into interoperability issues before. In fact, it's common even to see incompatible documents amongst Microsoft own suites, such as the inability for Office 2000 to save .DOCX files. This is yet another in a laundry list of complaints Microsoft is facing, along with a large fine they are appealing, in the EU.
Above all, the EU seems dead set on making Microsoft change their practices, whereas Microsoft doesn't want to see their various cash cows hurt in any fashion.