I love silent comics! They were my specialty for a while, as were children's picture books. They are especially good for shorter, more powerful/emotional works. They are also great for works focused on the environment or a specific ambiance, or, as mentioned, exploring surreal worlds. If I have a comic that is very environmental or dream-like, I will often make it wordless to create extra impact.
Expression and body language are important in dialogue comics as well, but silent comics are dependant on it. It is best to get several readers to look it over and make sure they know what is happening. Rework complicated scenes over and over to make sure you have achieved the best way to communicate it. People become frustrated if they can't understand what is happening, or if there are too many options for what is happening. You may also just have to accept that certain parts will have to be open to interpretation.
I find wordless comics have a stronger importance on quality of artwork. Readers will focus more on the art and composition, as there are no words to distract them. They have to read the story from the art, and therefore will notice if the quality is not there. I will not spend as much time as I should on my comic artwork, as I am trying to get the story out as quick as possible, but I do not have that luxury if I am doing it wordless.
The best, best, BEST work to look at for study and reference is Shaun Tan's The Arrival. It is a gorgeous book about immigration, each panel is absolutely stunning and the use of no dialogue also plays a part into the story (as it is about immigration the character cannot speak the local language, and we as the readers mirror the immigrant and must read the story without language). This book is used for study in the Sequential Art major at SCAD, as communicating a comic successfully without words will make you increasingly successful in a comic with dialogue.
I also suggest to avoid too many uses of symbols (such as surprise lines, question marks, exclamation marks, shouting line, etc.) as they can be considered similar to dialogue. They are giving a universal explanation, and therefore take away from a truly silent comic. But, they can be useful and fun, so decide if you want to use them or not from the start, so your comic can be consistent.