While it may be much more difficult for them to plot another attack on a high profile target, there is still much damage they could inflict. A wave of simultaneous attacks on much smaller targets, such as McD's or WalMarts, would be devastating.
I hate to make it sound trivial, it's not, but random McDonalds and stores being shot up isn't exactly new in modern American life. It wouldn't attract enough attention to be appealing, most likely. Remember the goal of the terrorist. It's not just random mayhem, they want to get as much attention as possible for their agenda in order to affect policy that meets their goals. Or the person doing it is just plain loony with wild delusions of grandeur but probably is so paranoid he or she doesn't work well with others, take your pick.
If you're talking abut a coordinated effort, then you're again talking about a level of coordination and sophistication that makes it much more difficult to pull off and avoid detection. If you're talking about not just a shootout but blowing stuff up, you add the element of obtaining, manufacturing and placing the devices. The more complication and conspiracy you add, the less likely it will succeed. Especially in the post-9/11 environment. Not impossible, but insanely difficult IMO.
I don't doubt that they could recruit enough people to walk into such establishments with bombs hidden and blow themselves up at the same time. It really wouldn't be that complicated, and quite easy to go undetected. The impact psychologically, and then financially, would certainly be felt. For whatever reason they haven't attempted to undertake such operations, I'm still surprised we haven't had some sort of successful attack.