Main differences I have noticed are,
• Previously the resistant coverage range (eg. 99/0) couldn't be mixed with anything other than mix tones. The new range has better coverage on resistant hair when mixed with other tones.
• If the resistant range (eg. 99/0) is used on hair that isn't resistant then the result will be deeper/darker.
• The new ME+ colours aren't as forgivable as the old technology when it comes to overlapping colour. Expect obvious banding if you end up overlapping your colour. The plus side is you become very neat at applying colour
• I feel like I have to use more colour to get the coverage with the new ME+ colours.