Raw honey has bits of wax, bee parts, dust, pollen, microorganisms, and the like in it. You have the most control in how you process raw honey, but you also have the most to do. Honey may be filtered, or blended, or even heat-pasteurized to make it clearer and less likely to crystallize. The more processed it is, the milder it is likely to be and the less character it will give to your mead. The processing also dissipates some of the honey's aroma.
Crystallized honey is normally acceptable for mead.
First, it generally indicates less processing, since one of the reasons for processing honey is to keep it from crystallizing. Second, it may be cheaper because it's less appealing to the average consumer. To re-liquefy crystallized honey so you can pour it, just heat it gently. Yeast prefer an acidic environment. Many other micro-organisms don't.