The honey you use is the most important contribution to the taste of your mead. Commercial honey has been heated for pasturization and to make it easier to squirt from bottles. The heating process is not done carefully to preserve the delicate smells of the honey, but to move the "product" as quickly as possible. Commercial honey is also usually a blend of several honeys (e.g. clover and alfalfa) to make for a consistent sweetness than for interesting tastes.
Natural, or unfiltered, or road-side-stand honey is usually unblended and unheated. You have more control over it and it will impart more flavor to your mead.
You will find that honey comes in many different shades. Generally speaking the lighter the color of the honey, the lighter the taste of the honey; the darker the honey, the more flavors it will impart.
You will need to choose a particular type of honey for the type of mead you want to make. For a traditional Mead you would probably want to use a darker honey for more flavor; using processed honey or a lighter honey for a traditional Mead would end up giving you something that tastes like sugared alcohol or have very little flavor. For a melomel you would probably prefer a lighter honey (even a processed honey) to allow the flavor of the fruit to come through. In general lighter honey for fruits and darker honey for spices seems to work well.
See also: Types of Honey
You say your honey has crystallized? You want to know if you can still use it? Sure you can.
Honey is mainly a supersaturated solution of glucose and dextrose. The bees raise the concentration to this level to ensure that wild yeasts will not cause the honey to ferment. Because it is supersaturated it will always crystallize, if given enough time. Note that it is just a change of phase. There is no water loss. Indeed, if you leave honey open to the air for any length of time it will absorb water, not loose it.
The tendency to crystallize differs from one type of honey to another. This is a very complicated subject. Usually the most important factor is the glucose to dextrose ratio but the presence of traces of certain proteins can inhibit crystallization. The other important thing is the presence of seeds on which crystallization can start. Often particles of pollen act as seeds. As a consequence, natural honey is more prone to crystallize than processed honey which has been well filtered (as well as pasteurized, blended, etc.)
All you have to do is heat the crystallized honey back up and it will revert back to it's liquid state. You may either heat it up in the microwave or put it into a pot of hot water to dissolve it (the later being my preference). When making your weight measurements for honey treat the weight of the crystallized honey the same as you would the liquefied form. In other words X oz's of crystallized honey is equal to X oz's of liquid honey.
The following is the definition of honey as given by the National Honey Board
HONEY DEFINED The National Honey Board has taken on a daunting task, defining honey! It may come as a revelation to some that honey has had no official definition. On further reflection, however, it is not surprising. Imagine trying to come to grips with an adequate description of a natural product that is infinitely variable. After a year-long discussion with industry representatives and others, the Board's Product Research and Development Committee has "...a document that describes and defines our product." The actual words used in the official definition were approved October 9, 1993 and are subject to review every two years: "Honey is the nectar and sweet deposits from plants as gathered, modified and stored in the honeycomb of honey bees." That's not all of course; several honey categories and other considerations are also discussed in the three-page definition document. These include honey composition, types of honey, designation of honey sources, forms of honey, honey products, grading and methods of analysis. Composition of honey is perhaps the most problematic topic to deal with in defining the product. Given this fact, the Honey Board has chosen to list an average, range and standard deviation for major constituents. The standard deviation is an estimation of how variable each specific item is. The higher the number, the more difference that can be found among various kinds. The standard deviations themselves show a large range from 70.9 (total protein is extremely variable) to 0.126 (fructose/glucose ratio is more consistent). The following are the actual numbers: Average Range Standard Deviation Fructose/Glucose Ratio 1.23 0.76-1.86 0.126 Fructose,% 38.38 30.91-44.26 1.77 Glucose,% 30.31 22.89-40.75 3.04 Minerals (Ash),% 0.169 0.020-1.028 0.15 Moisture, % 17.2 13.4-22.9 1.46 Reducing Sugars, % 76.75 61.39-83.72 2.76 Sucrose, % 1.31 0.25-7.57 0.87 Total Acidity, meq/kg. 29.12 8.68-59.49 10.33 True Protein, mg/100g. 168.6 57.7-567 70.90 Although the percentage of fructose and glucose constituents are about the same in honeys, glucose is more variable with a standard deviation of 3.04 as opposed to fructose's 1.77. Fructose is the major sugar component which provides the extreme sweetness in honey. This sugar also reduces possible crystallization in the product; Florida tupelo honey is well known for its high fructose content and tendency not to "sugar." The percentage of sucrose in honey has a larger range than might be expected. Citrus honey from Florida has been rejected in some international markets because of its relatively high sucrose content, which is also thought to promote crystallization. Obviously, some honeys are much more proteinaceous than others. Perhaps this will result in some interesting claims by producers in response to the well-known declaration that honey is nothing more than carbohydrate! Of all the numbers presented above, those with reference to percentage of water are perhaps most significant to honey judges. The standard for moisture content in honey shows has traditionally been 18.6%. Does the upper bound shown in the official definition (22.9%) mean that judges will have to accommodate honey in shows with what heretofore was considered an unacceptably high moisture content? In any case, this information will require changes in ENY 129 "Honey Judging and Standards" and ENY 130 "Moisture in Honey," available from this office in limited supply. The official definition does incorporate current U.S. standards and grades of extracted and comb honey which are quoted at length in the above fact sheets.