For me it's the size of the blockchain. Even with the exponential improvement of storage, computing power, and bandwidth. Something that big simply won't scale and is not sustainable for the 100's of years that is expected of Bitcoin.
That is true as well, how big is the blockchain now? I quit trying to use the bitcoin.org client, It takes too long to download, I heard even in high speed vps it takes about 3 days to download and that is insane.