You gear looks fine. I would tend to say it's a technique issue. Before I get to that there are a few things about your gear. Most people are gemming straight stamina and reforging for expertise, mastery or hit. Gemming for resiliency is bad. Mark of supremacy is not a tanking trinket. Since you asked about threat I will stick to that topic. Your expertise is slightly low, unless you eat expertise food. It needs to be 26 because dodged attacks create no threat. Are you getting rage starved with no points in shield specialization? In terms technique, are you charging in using rend, then using thunder clap and shockwave? Do you use cleave on multiple mobs? Are you using shield slam and revenge on every cool down? Do you rotate your targets? Aoe is not a warrior tanks bread and butter, but using rend then thunder clap, shockwave, and cleave is usually enough to hold aggro on multiple mobs. Unless you are doing heroics with mega geared dps who don't give you time to build aggro at all you should be able to hold aggro. Single target threat is really what warrior tanks do best imo. Shield slam, heroic strike, revenge, devastate is an awesome single target rotation.