I've been following all the legalization news closely since I live in Denver. The FUD from opponents of legalization are that it will negatively affect children since access will increase. The 'hey mister' won't be only for alcohol anymore but for cannabis too. Many edible products seem to be targeting children with products like gummy bears, hard candies, brownies, sodas, etc. This would increase the likelihood of accidental ingestion but also seems to be purposefully targeting children's interests similar to cigarette cartoon advertising. The issues around advertising, specific products, and increased access for children most likely will be an ongoing problem that will require additional legislation to resolve.
Second there is a big fear of increased DUI. The framework for legalization required that there be a penalty for DUI, which has been pegged at 5 nanograms of THC per millimeter of blood. This has its own problem, primarily that somebody who is no longer under the influence can still test positive if they are a heavy user. Officers need better tools at their disposal to determine if a driver is actually a danger while driving. This seems no different than legal pills that people also drive while under the influence. This might be a technology problem since there is no way to tell if a person is under the influence of cannabis.
The third big fear is that organized criminal elements will be attracted to the legal market in an attempt to launder their black market sales. There was a big bust of legal grows and dispensaries earlier this year which was supposedly related to cartel sales out of Miami. This seems like the real reason the DEA should be involved in cannabis legalization. So they can determine who is playing by the rules and who is trying to subvert them.
Without much quantitative data, the qualitative affects of legalization seem to have a positive impact on people's lives and the general communities around Denver in my personal opinion.