The Smith machine is rated for 450 lb max loading. The bar weight is approximately 15kg. It has a larger diameter of 35mm versus a standard barbell that has a diameter of 28-29mm. REP engineering chose to use a 35mm bar diameter to allow for the bar to be both strong and lightweight. With a 15KG start weight and 450LB weight capacity, a standard solid barbell would have been way too heavy. With a larger diameter, REP is able to achieve both a lighter start weight and the strength needed for a high-performance Smith machine. There is no counterbalance. REP has worked to lighten the bar enough so that a counterbalance was not needed and would only increase complexity and price of the product.
Compared to a standard barbell, the 35mm shaft diameter may been like it may be hard to grip. For the majority of exercises that involve pressing or the barbell resting our your back, the larger diameter actually feels better, as it helps spread the load out and lessens the pressure. For pulling movements, due to the larger diameter, the barbell will be slightly harder to hold onto than a standard barbell. However, for most Smith machine exercises, we don't think most people will have issues.