Nucleation and growth of nanoparticle crystals is often difficult. Recent progress has been made by linking dilute solution properties, such as the osmotic second virial coefficient, B-2, to equilibrium phase behavior of nanoparticle suspensions. These studies show that, at the same values of B-2, broad classes of nanoparticles have similar phase behavior. However, crystals nucleate from supersaturated solutions at greatly different rates. Links between strength of attraction and rates of nucleation have not been established. Here, we investigate the role of solution variables on the kinetics of nanoparticle crystal nucleation through measurement of the induction times for crystal formation. With silicotungstic acid and lysozyme, the effects of temperature, ionic strength, and additives such as poly(ethylene glycol), glycerol, and diols on crystal nucleation times are studied. The effects of the additives on the strength of attraction are characterized through measurements of B-2 and solubility. Induction times for crystal formation are interpreted using classical nucleation theories resulting in estimates of the surface tension of the critical nucleus. We find that the surface tension and thus the free energy barrier to the formation of a critical nucleus is strongly correlated with B-2/B-2(HS) where B-2(HS) is the second virial coefficient associated with the hard-core interactions. In addition, we find that the prefactor is substantially smaller than expected from classical theories and also shows a strong correlation with the strength of nanoparticle interactions.