The first is that you're essentially creating a new class of roadway users, "vulnerable users". Overall this is a good thing. A driver of an automobile can do significantly more harm and therefore, should have more responsibility, much in the same way we hold commercial truckers to higher standards of conduct than we do drivers of private automobiles. The downside of this is, that by creating a separate class of users with differing levels of responsibility, we can inadvertently lay the groundwork for giving different rights to different types of users. This is less of an issue with pedestrians because they are already a separate class of roadway users, i.e. they are not vehicles. With bicyclists, however, there has been a history of trying to get them banned from certain roadways, citing safety as a concern (i.e. it's for their own good). There have been attempts in a number of states as recently as 2009: http://www.kansascyclist.com/
The second issue with vulnerable users laws is that you have to be both careful and specific in terms of how you define a "vulnerable user". Is a motorcyclist a vulnerable user? Are vulnerable users synonymous with non-motorized roadway users? With the proliferation of electric-assist bicyclists and neighborhood electric vehicles like golf carts there is, and will continue to be, an increasing variety of vehicles on the road. With the population aging, these types of vehicles will become more and more common as many older citizens may no longer be comfortable driving standard automobiles and switch to these small less powerful vehicles. This is already happening in a lot of retirement villages in the sunbelt. If a 20-year-old cyclist hits an 80-year-old driver of a golf car, who is the vulnerable user? Are they both vulnerable users?
The third thing to watch out for, or at least be aware of and prepared to address, is the fairness issue. One complaint I've heard a number of times is that vulnerable user laws can create legal rulings that are very counter-intuitive. The most commonly cited example goes like this: Drunk driver A hits and kills another motorist while drunk driver B hits and kills a cyclist. Under some vulnerable user laws, driver B would face a more serious penalty for his crime than driver A, even though both drivers committed the same act with the same end result. The punishment varies depending on the type of vehicle your victim happens to be driving.
That last thing to be be careful of is assigning too much blame to drivers in cases where cyclists or pedestrians may be overwhelmingly at fault. Penalties for drivers who hit and injure cyclists and pedestrians certainly need to be stiffer and more regularly enforced, but if penalties or the assignment of fault is draconian, or is perceived as genuinely unfair, the law may ultimately be ineffective and generate a backlash. Juries tend to be sympathetic to drivers because everyone on the jury has typically driven a car and can put themselves in the driver's shoes. They may or may not be able to relate as freely to the cyclist or the pedestrian.
So with all these issues, why have a vulnerable users law at all? Why should cyclists and pedestrians get special protection under the law? The simple answer is this: driving is a privilege, walking and biking are rights. In order to drive a car you have to have a driver's license issued to you by the state you live in. This license can be revoked by the state for any number of reasons, largely because reckless drivers can, and often do, inflict a tremendous amount of harm. People ought to have the freedom to choose how they want to get around, and should be able to do so knowing that they'll be adequately protected by the law.