The biggest issue I see with Paul is the across the board tax cuts. After doing a bit of research around his sites Issues page, he wants to cut taxes everywhere.
While this might seem like a good thing, the money has to come from somewhere. If we want a government that can function effectively, it needs money. Pure and simple. Tax cuts have and continue to  cut funding from programs that need proper funding to achieve their goals.
A good example is No Child Left Behind, or NCLB. NCLB, President Bushes education bill, set very aggressive goals for closing "achievement gaps" between different ethnicities within schools, and created a system of evaluation for schools, that forced them to conform to national standards or face restructuring or closure. However, the amount of money that has been appropriated is far less than is necessary to fully implement the bill and many states are suing the federal government to get proper funding to accomplish the goals of the bill.
If enough money was available, NCLB could possibly function as intended. However, it has been consistently underfunded and is causing many schools to be shut down, especially in areas of large poverty such as the Native American tribal reservations of Montana. In this case, the lack of funding has forced the destruction of the school system in these areas, and makes an already bad situation worse.
As the application of NCLB demonstrates, funding is necessary to run programs; and without the taxes that Paul wants to cut, more programs will suffer and will continue to suffer shortfalls. That is why I am not  a fan of Paul.