Some douchebag with an overly manicured beard and shit between his ears managed to badly injure about 30 people he'd never met. He tried to hurt far more people, but his incompetence prevented him from doing that. He was also captured alive, so he'll probably be spending the rest of his life in prison wishing he was dead.
Another asshole went to a shopping mall in Minnesota and started stabbing strangers for no good reason. An off duty cop who happened to be carrying a pistol shot him before he could kill anyone.
If you look past the fact that these guys are/were repulsive sociopaths, one thing that is worth thinking about is that the bomber probably spent a few hundred bucks on his attack. Knives are so abundant that they might as well be considered to be free.
A few years back John Robb (he wrote Brave New War, you should read it) described what he called a terrorism tax. Essentially, Joe and Jane average begin to scale back on various economic activities because they're worried that people will kill them. An article on CNN states that one billion dollars are spent yearly to keep New York safe. Granted, in a city that big the per taxpayer cost probably isn't exactly huge in and of itself.
This begins to be a bigger problem if people stop going places because because they're afraid that someone will try to murder them because it just happens to be their unlucky day. Revenues go down. People lose their jobs. People don't go out because in addition to being afraid, they're broke. This creates a feedback loop where the weakened economy creates an even weaker economy.
We also have to consider that with a problem like this, the cure may be worse than the disease. My wife is a food nerd, so we have a couple dozen different kinds of knives in the kitchen. I have a variety of knives for general use, fishing, camping, etc. Where I live both black and smokeless gunpowder are sold at a many stores. Even if it was to be banned, black powder is eighth century technology. People are going to make it if they want it.
The only sensible response to these little pissant terrorist attacks is to treat them as just another risk. If I die of unnatural causes it'll most likely be on my commute to work and back. This doesn't stop me from jumping in my car every morning, and it doesn't keep me from going ninety miles an hour every now and again. It does cause me to wear my seat belt and to not drive if I've had too much to drink. The equivalent response to these little Junior Terrorist Club asshats would be to do something like get first aid or self defense (armed or unarmed) training, along with the requisite equipment.
Using your head is increasingly important in our modern world. Your instincts developed in a world that no longer exists and often provide bad information. Going with your gut leaves you vulnerable to people who are actively working against you.