3) It doesn't compare well against existing systems in terms of privacy, speed, or transaction cost.
Privacy? But making the transactions private is about as you can get. Without physically accessing your computer while you are making transactions the only information an external entity can access about you is "there has been an encrypted connection established from your location to an arbitrary and potentially unrelated computer on the internet".
Yeah, I know. That was the part that stood out the most for me -- the electronic funds transfer network he's using cannot possibly be offering the privacy that Bitcoin does, as long as his name his tied to it and a central arbiter sees all the transactions.
Not claiming that critique was flawless, just that it's (for the most part) intelligent and based on familiarity with the system.
Annie Lowrey discusses Bitcoin in Slate. No clear thesis, but important that it gets attention there. She gives a general overview, with emphasis on its benefits to fringe elements on society, and gives quick attack at the end. The attack seems misinformed, but it links to something more interesting, specifically...
A technical critique by Victor Grishchenko, PhD, who was mentioned here in the context of causal trees. He describes a few problems he sees with Bitcoin:
1) Asymmetry favors attackers, in that it takes a lot more effort to check for double spending than to attempt a double-spend, eventually requiring "supernodes" that have disproportionate influence over the network.
2) It needs to continuously spend spend cycles to stay free from attackers. He then describes an attack I don't quite understand that involves holding on to a discovered block and then broadcasting it at just the right time
3) It doesn't compare well against existing systems in terms of privacy, speed, or transaction cost. (I found this questionable because the system he's comparing it to is still subject to warrants, and Bitcoin takes significantly less time -- 1 hour or so -- to ensure a transaction than the wiring transfers Grishchenko discribes.)
Finally, he credits Bitcoin in being advantageous similarly to Bittorrent: the latter was clumsy and complicated compared to regular downloading, but could perform well enough in a niche niche to force change in the broader markets.