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If it ain't broke, don't fix it. Especially when it comes to MS software. >>
Very well said.
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What features does IE5.5 have that 5.0, 5.01 or 5.1 don't have? >>
They are mostly developer-oriented.
i.e.:
Enhanced visual filters (the DirectX transform thingies)
You can change colours of the scroll bars (oooh! big deal

)
More enhanced HTML editing features (i.e. my text editor)
I believe that it added a few more CSS attributes also
Print Preview (already mentioned)
It handles CSS and DHTML slightly differently (more headaches for me) - feature of bug?
Nothing major, as far as the end-user is concerned. Even the XML parser that ships with IE 5.5 is the same version (or maybe 0.1 version ahead) than IE 5.0.