Guns.
I think the left is extremely naive on gun issues. We absolutely have a problem with gun violence in our country but I think it's bigger than guns. Canada has a ton of guns but they aren't killing people anywhere near the rate we are. Yes, banning "assault" rifles and high capacity clips will help reduce the number of lives lost in mass shootings but I would rather get to the problem of why someone would be involved in a mass shooting instead of going after the tool they used. It just seems like a bandaid approach and an appeal to emotion rather than fixing the issue.
To me, the genie is out of the bottle with guns. You can't undo the technology. You can ban them but they are still out there and people will always get them. A quick look to UK tells you what happens when people don't have access to guns but still want to kill each other. They use knifes, swords, cars, bombs, etc.
The guns restrictions currently in place I feel are good enough. There's only so much you can do before it gets heavy handed. It's like murder or drunk driving. It's illegal to murder someone or drive drunk. You go to jail for either yet people still murder and drive drunk. There's not much else you can do at that point other than to educate and discourage people. You don't ban cars because that's what they use to drive drunk.
Most of the gun violence is coming from poverty, gangs and domestic issues. Focus on those issues if you want to stop the mass majority of gun violence. You are never going to solve it all though and I think that's where the left is extremely naive.