To expand upon a comment from an earlier post, there are three goals when making a special effect.
1) Convey what the effect represents.
This is the most important goal. If you create a model city, it needs to look enough like a city that the audience knows that it’s a city when they see it.
2) Look cool
Try to make it something interesting to look at. It helps keep the audience entertained. (Of course, I don’t recommend having a movie where the effects are the only thing that will entertain the audience.)
3) Look realistic
This is what people mean when they say they want effects that don’t look like effects. The effect should feel like a real part of the world of the movie. This is often the most difficult goal to meet.
Achieving goal 1 is crucial. Sometimes goals 2 and 3 will be at odds, and you may need to trade one for the other. The relative importance of those two will depend upon the tone of your movie. If you’re trying to make a retro-future pulp adventure you might emphasize 2 more than 3, while if you are making a complex psychological drama you’ll stick with 3 over 2.
In my own work, I’ve usually achieved goal 1 and sometimes 2, but rarely goal 3.