While there may not be One True Definition, and there's room to agree and disagree, there are attributes people probably could agree are tasteless and help qualify a house as a McMansion:
* No clear architectural style, or a haphazard/incoherent mix of styles
* Brick or stone facades in front, and cheap siding on the (less visible) sides
* Non-structural arches and columns
* Oddly placed/sized windows
* Multiple roof lines, chimneys and dormers
To the extent that a "Recently built house in a more or less modern style" incorporates these elements, it can be considered more or less a McMansion.
The point is, with the effort and money spent, the houses are not even necessarily nice to live in when the designer just adds a hodge podge of features. The critique may sound elitist but the actual defects are concrete once you notice them and not just 'accept and adapt' to them.
There is a fundamental difference between a 17th century house with wing added in the 19th century and garage added in the 20th century, and a 21st century house trying to mimic that look.
* No clear architectural style, or a haphazard/incoherent mix of styles
* Brick or stone facades in front, and cheap siding on the (less visible) sides
* Non-structural arches and columns
* Oddly placed/sized windows
* Multiple roof lines, chimneys and dormers
To the extent that a "Recently built house in a more or less modern style" incorporates these elements, it can be considered more or less a McMansion.