NSA, China, Russia, most governments really, and possibly a lot of banks, could do a 51% attack and shut down transaction processing... or do double spends, but shutting down all transaction processing seems more damaging. This won't ever change, unless Bitcoin really causes governments to lose most of their power and money.
Edit: gavin has a plan (http://gavintech.blogspot.com/2012/05/neutralizing-51-attack...) but my guess is if they did the work to start an attack in the first place they'd have planned for gavin's plan and acquired enough older coins to foil it. It'd be nice if more thought went into other ways to thwart a possible attack but I'm not sure there really are any. Most people seem to discount and completely avoid the issue.
Edit: gavin has a plan (http://gavintech.blogspot.com/2012/05/neutralizing-51-attack...) but my guess is if they did the work to start an attack in the first place they'd have planned for gavin's plan and acquired enough older coins to foil it. It'd be nice if more thought went into other ways to thwart a possible attack but I'm not sure there really are any. Most people seem to discount and completely avoid the issue.