I actually quite liked Tristan's argument.
[attr, attr2] feels more like what it is, an annotation. It does nothing, has no effect on the declaration on its own.
the @attr syntax looks like existing attributes, and with that kinda comes the presumption that they actually DO something, affect the code generated in some way.
For that reason, I buy the argument that [attrs...] being visually distinct makes more sense, more like what it is.
Perhaps it should be termed 'annotation' rather than 'attribute'?