51% attack: This would require extensive infrastructure, even for an attack when we were at 21th/s. this would be expensive and time consuming; and while there certainly are ways to hide it, would increase the attack footprint with specialists required to set up and maintain the cluster, which runs the risk of a leak, giving bitcoin more credibility. they would also, as I understand it, need to amass a significant amount of old coins in order to continue the attack, which would only last as long as their reserves of coin.
Attack on companies offering bitcoin related services: persistent, coordinated attacks on web sites offering bitcoin services would reduce confidence in the sites, making it seem like they are run by a bunch of amateurs, reflecting poorly on the bitcoin marketplace.
Propoganda: This one is fairly obvious, and doesn't really need to be actively funded. The world governments and CBs have already invested countless dollars and man hours into brainwashing the masses. The media coverage of bitcoin is a shining example of how difficult it is for the general public to comprehend the idea of a free market and monetary system. People I have great respect for surprise me time and time again with their views of what money is.
Market manipulation: The bitcoin market is brand new, small, and fragile. It would not take much money, money they can (and do) print by the truckload every day, to orchestrate a massive pump n dump to shatter consumer confidence. The beauty of this attack is its difficulty to detect. The community would be so focused on the growth of the market and increased value they wouldn't notice the slow accumulation of wealth spread over multiple accounts. All it would take is a sudden dump of all the coins amassed to tank the value, destroying confidence in the market.
Illegal: this ones tricky due to the nature of bitcoin. It would take an unprecedented amount of coordination to make it illegal in every country, which would at best only slow adoption. History shows how difficult it is for governments to eradicate something if society demands it.
Regulation: this is a better alternative to making bitcoin illegal, though difficult given the nature of bitcoin. The easiest way would be to regulate the exchange into and out of bitcoin and fiat currencies. Vendors that accept bitcoin could be taxed on all purchases made with coins. I'm sure the government can come up with creative ways to ensure they get a slice of every pie we buy.
I believe we are currently under attack by a mix of these approaches combined to be the most effective. Who stands to gain by DDOSing bitcoin services? Who has the money to buy million dollar plus market value chunks over and over again? Greed, fear, and the inability of the current market to sustain the level of trading certainly had a large part to play in the recent crash/correction/whatever you want to call it, but what if it was something deliberate? What if someone knew the gox trading engine was maxed and they could cause a panic sell by lagging it out? How difficult would it be to spend $100 million in fiat to pump n dump the economy. Not to make a profit, but purely to destroy it? Do I have my tinfoil hat on too tight?
I completely agree. I view these attacks as a good thing. They show us where the weaknesses are and how we can reduce or eliminate them. A single exchange carrying the fiat/btc exchange rate weight of the entire network has been a weakness from the start. Hopefully we'll begin to to wider adoption of other exchanges and new ones popping up. Once the community realizes centralization is a weakness we'll be well on our way to creating the resilient infrastructure necessary to bitcoins survival