A lot of level designers don't follow this idea, but eyecandy is absolutely nothing without theme. Theme is basically the subject of the level, and the eyecandy serves as the content. Writing a story without a title is meaningless - so why does some people think it doesn't matter in level creation? Simply put, your chosen theme should determine what kind of decorations your level will have.

There are some occaisions when using repetitive eyecandy can actually work out. For example, cooba and Ragnarok's Rainforest Revelry uses a complex eyecandy method which gives the feeling of density like you're actually in a jungle, which makes perfect sense as the use is justified by the theme. This type of eyecandy approach would not work in something like say a Castle level, as logically an "artifical" level would have more finesse in it's design - you don't see totally random house designs now, do you? The tiles you use also help create a theme. Sometimes it's not always a good idea to use all of the tiles in a set, and instead focus on certain aspects and making them stand out, if my level Condemned is an example of this.

Theme can also justify gameplay as well, for example an indoor level would likely have a more enclosed design built out of paths and corridors, whereas an outdoor theme would logically have more open spaces. It's not a general rule of thumb though, there are exceptions and it can be interesting to see the two styles mixed and/or reversed.