V2 is actually the minimum speed at which, with one engine inoperative, a specified gross gradient can be achieved (3% for 4-engined aircraft, 2.7% for 3-engined and 2.4% for 2-engined), subject to it being not less than the factors that FE Hoppy states above.
However, there is one other consideration that must be taken into account; Vr must be not less than Vmcg. Therefore, if Vr is determined by Vmcg then V2 may become purely the speed which is attained at screen height following rotation at the specified rate; this applies especially at low TOW. This needs to be remembered in older aircraft where flap retraction speeds are sometimes based on V2 (747-200 for example). This procedure is to give a margin above the stall. However, if V2 is determined by Vr equalling Vmcg then the flap retraction speeds will be greater than is needed for stall margin.
I know that much of this reply is a little off the original question but it is important to realise that if you are below V2 then your climb gradient will be reduced.