When I first started doing scrappy quilts, the teacher said that you need contrast. Most of us have a lot of mediums in our stash and as a result we put too many pieces of the same value. Having lights and darks is really good provides definition for the pattern. The other thing she said is to have a place to rest your eye. In the pattern you sent the link to, the dark sashing provides the resting place for your eye. Each triangle pair has a lot of contrast the light triangles are significantly lighter than the dark triangles. These elements make the quilt striking.
Your quilt gets muddy from lack of contrast. When there are a lot of mediums or the mediums are either too close to the darks or lights that they are paired with then the quilt has a more monotone look. One classic technique to determine if your values have enough contrast is to take black and white pictures of the fabrics next to each other until you can look at them and see if there is enough contrast.