First, through this whole process you'll need to have your budget in mind. Remember, it’s not just for the kitchen cabinets themselves but also hardware and installation that you will be paying for. This is especially important when shopping for kitchen cabinets online as the price is not always listed. Once you have a solid budget for your kitchen cabinets, you’ll know what is within reason.
If you're replacing existing cabinets, note what you like and don't like about your current arrangement. If you have limited space, consider a compact, space efficient unit that does double duty, like the kitchen storage and trash bin combo which saves valuable floor space. Do you have areas that are hard to reach or are not being used for some reason? There are now interior options available (such as carousels and custom drawers) that can help you gain more storage you may not be currently accessing.
With new and remodeled kitchens, you'll need to layout your design for your kitchen cabinets. You will probably need to consult with a professional designer. Keep in mind your daily routine; what may look nice as a layout on a showroom floor, or when looking at kitchen cabinets online, may be a hassle from a practical standpoint. You'll also want to keep in mind where you want your major appliances and make sure your outlets and cabinet layout will allow for this. This is a good time to consider any additions for both storage and counter space, such as an island, that you may not have had previously.
When you go into a showroom or shop for kitchen cabinets online, the primary thing to look for is the overall construction of the kitchen cabinets, not just the finishes (find a solid type of cabinet, and the finish you want can probably be special ordered). Inspect potential cabinet choices closely, looking for issues at the joints, shelves, and hinges. Make sure that drawers open and shut smoothly and that the shelves are thick and don't buckle under the pressure of you pressing down. Check to see if the interior of the cabinet is particle board or real wood.
Glass doors are really nice to showcase items that are significant to you and look nice to display. A good way to judge whether you would want them on all of your cabinets however is to open up all of your existing kitchen cabinets and pretend they were glass-faced. Also, pick a finish for your kitchen cabinets that will go well with multiple wall and floor colors. You'll most likely be changing those before changing the kitchen cabinets again.