It would destroy Bitcoin's reputation as impartial, unimpeachable, immutable money. Not only would it turn governments against us, it would also show people that money they receive could be taken from them if they lose in the court of public opinion. It would be like a giant billboard saying BITCOIN: YOUR MONEY AT THE WHIMS OF THE MASSES.
And some people say there's a clean line we can draw in the case of the feds. How about legitimately seized bitcoins, from real criminals? How about those of the big banks? How about large companies like Apple and Walmart, that have close ties with the government just by virtue of their size? How about pirateat40's stash? Why not anyone with a Scammer tag on bitcointalk? Heck, why not annul the bitcoins of all taxpayers, since they feed the government by not resisting?
This is the most dangerous idea to come about in a long time. Please, please, please think this through.
Truly, a mind ahead of his time.