Width. There are extremes on either side of the spectrum: The lobster bib sized 4.5 incher on one end, and the pencil-thin trendy tie on the other. I like to practice moderation when selecting a tie. Too wide and it's outdated and out of style. It also adds a visual element of width/girth that you probably don't want. The ultra-narrow ties are trendy and will likely cause "what was I thinking?" types of sentiments in the not-too-distant future. Better to stick to the safe zone of about 2.5 inches to 3.5 inches. An inch may not sound like much of a difference. It is. 2.5 inches wide is too narrow for some conservative types, but is a nice, contemporary look. 3.0 is a great middle ground: it looks contemporary without coming off as trendy. 3.5 is a classic choice for guys who want a bit wider tie without looking like it's been in their closet since 1997.
Remember: your suit lapels should be around the same width as your tie, and your shirt collar should follow the same rules as well. For consistency in your look, narrow tie>>narrow lapels>>narrow shirt collar, and the same for wider ties, lapels, and shirt collars.
A great tie has a good shape. After the tie is knotted, the tie gradually gets to it's maximum width. Skinny at the top and wide at the bottom is not good--while there should be some difference between the width of the tie up by the collar and the width at the bottom of the tie, it's not a drastic change.
Next up in part 2: How to pick your knot.
Some good examples of width: Note how the width of the tie matches the shirt collar. When knotted, these will look consistent, regardless of the spread of the collar.
A few bad examples: When knotted, these ties will be too narrow compared to the shirt collar. The smaller knot yielded by the narrower tie will look oddly small next to these long collars.