I see what you all mean with the flaws, so I guess it'd be a lot easier to just use the D&D system for this.
Moving on, an important thing that needs to be addressed is how the job system will work. Will the reward for a job be split equally, or will each member receive the listed amount? The former will probably be more balanced(More members means less reward, but easier), but the latter would reenforce teamwork(More members makes it easier without reducing the reward). The amount could be determined simply by the job creator's will, or it could be a ranking system(Each "tier" will have a reward range, which will generally correspond with its difficulty).