I was engaged in a discussion at Think Defence, and want to salvage at least one part of the effort for my own blog:
SO define a modern heavy tank and a modern medium tank then?
A modern heavy tank was designed* with the ability to withstand all hits on its frontal (at least turret) armour and with the ability to penetrate all hostile front armour in mind. Mobility was a secondary development concern.
A modern medium tank was designed* with the ability to defeat all tanks and the ability to withstand almost all** hits on its frontal (at least turret) armour in mind. Substantial sacrifices were made to enhance mobility***.
*: Not necessarily with lasting success or even mere initial success.
**: Including common, but not the most powerful AT munitions.
***: This is rather about soft soil performance and choice of bridges than top speed.
These (unofficial) definitions show why I think mediums aren't at a major disadvantage: The heavies lose their edge to technological progress quickly, while the medium's advantages are more persistent.
edit: These two definitions are concise descriptions of what I observed how others separate the two tank categories. Well, save for those people who only look at weight. I did not make this up by myself.
related: TK-X video (about the new Type 10 medium tank)