For multiplayer:

They're both important, but gameplay is more important. Also, people have always made their first judgments based on looks rather than how it plays. That's just how it is for most people. Gameplay is something that takes a lot longer to "sink in"... unless you are a good player, or a good level maker, it's hard to recognise good gameplay just by looking at the level. You have to play a few matches in it first, and that's a lot of work just to write a J2O review.

I also agree with Grytolle regarding obstructions. I have always tried to limit my use of layers 1-3 because it's usually just annoying if you block the player's view. Same goes with lighting.


For singleplayer:

Eyecandy is probably about equal to gameplay in terms of importance... you can have an extremely simple, easy level that looks beautiful and has a good story, and it will be a great SP level. However, a project should probably have a good balance of gameplay, eyecandy and story, rather than putting everything into one area. It's just that SP is really a lot more about the overall "presentation", rather than just how the level plays. If an SP level is ugly, I think it's fair to say it's bad. With MP, it's not as fair to judge based on looks.

An example of this is that elevator level in TR2. The elevator part is really what makes that level what it is, and it's such a cool effect. That alone makes it a good level. Without that part, it would actually be a fairly uninteresting level in terms of gameplay.

Also, in SP I think you should be allowed to break the rules about obstructive eyecandy. Obstructions can actually be an element of gameplay in SP, if they are used well. For example, you can use darkness to good effect, or hide enemies behind scenery (bats are good for this because they make a sound), or have secret areas. There's a very fine line between good use of obstructions and unfairness though.