A good model is one that is thought through with great detail and logic. Any model that deals with intrinsic valuation such a DCF is based off numerous assumptions and the modeler must be able to back up their assumptions and logic when asked. A good model is one that can be explained with sound logic and is relatively conservative with assumptions and drivers. If the modeler were to make a certain driver more aggressive, it would be crucial that they would be able to relate it back to macroeconomic trends or other factors to explain that logic or assumption.