GLUTEN is the protein(s) found in wheat. Celiac Disease is an autoimmune disorder in which gluten causes the immune system to attack the small intestine. People have several health problems because of Celiac Disease but one of the main issues is difficulty digesting nutrients due to the damage the immune response causes to the lining of the small intestine. Experts estimate that 1 in 100 people have the disease worldwide. Many people have it but aren't diagnosed yet because they haven't sought care of they've been misdiagnosed.
So what's the big deal about gluten? To those that are intolerant or have Celiac Disease, gluten can cause a cascade of autoimmune response. Even without the formal Celiac Disease diagnosis, a person with intolerance or allergy can have joint pain, headaches, fatigue, weight gain, swelling just to name a few symptoms. Many times these symptoms are part of someone's life for a long time so they're accepted as "normal." Others just call their symptoms IBS because they can't quite pinpoint what's causing their symptoms.
Is this gluten intolerance and allergy just another fad? Unfortunately I don't think this one is going anywhere. Autoimmune diseases and allergies are becoming much more prevalent in our society. And in fact, if people come to me asking what they can do to help their digestive symptoms improve, I commonly start by asking them to eliminate gluten (and/or dairy) because of the common sensitivity to the proteins found in these sources.
I don't think gluten is something that needs to be avoided all the time by EVERYONE. I think it usually does your body good to go some periods of time without gluten because that naturally cuts back on how much processed foods you're eating. When you eliminate gluten for about one month and then add it back into your diet, how your body responds will help you know if you're intolerant of gluten or not.
In summary, gluten is a protein that some people have an allergy or intolerance to. It's a big deal because if gluten causes an exaggerated immune response in your body it can negatively impact many body systems. And no, unfortunately not just another fad.