- the difficulty of finding available addresses is quadratic to the number of unspent transactions
- someone can slightly change the transaction so it's still valid, but have a different transaction ID - that can mess up clients and break chains of unconfirmed transactions
I would write it myself, but there is far better explanation here on reddit
http://www.reddit.com/r/Bitcoin/comments/1x93tf/some_irc_cha...
basically:
- the difficulty of finding available addresses is quadratic to the number of unspent transactions
- someone can slightly change the transaction so it's still valid, but have a different transaction ID - that can mess up clients and break chains of unconfirmed transactions