The key concept of a good day on the mountain is that you are Not Cold, which is not the same as being Warm. Comfort is a tricky condition to define because of how widely it varies from person to person, and this is very evident when considering ski pants.
For your ski pants to be comfortable, there are some concepts you'll need to take into account:
Moisture management: Your leg muscles will be working overtime while you ski, and even just walking around in your boots. The heat generated will probably lead to some sweat, and for most people, sweating inside your clothes is not comfortable.
If your clothing system is actively working to move moisture away from your skin and out, you'll have less 'damp' to deal with. Synthetic base layers next to your skin hold less moisture, move it away faster, and insulate better, even when wet. The breathability, or ability to allow water vapor through while keeping wind and liquid water out, of the fabrics will keep you comfortable and reduce the adjustments you'll need to make over the course of the day. Vents may be built in as pockets or in the thigh areas to rapidly move moist air out.
Wind/weather resistance: Any waterproof material is also windproof, because it won't allow air to pass through. By keeping the wind out as much as possible, you won't lose that precious warmth you've worked so hard to generate. Waterproof is very important when dealing with liquid precipitation, which snow becomes when you land on it and it melts, and so you'll see waterproof/breathable fabrics used in the shells of most ski pants. However, the designers expect that you are not playing in the rain, so only, but instead 'critical seams' are sealed, so the seat will be completely waterproof, but not the entire garment.
Insulation: Bulky materials are inherently warmer than thinner ones because they insulate better. However, if you are moving much at all, bulky clothing or multiple layers can be very frustrating, and you will get too warm, starting a hot-sweaty-grumpy cycle. Keep in mind how much your legs will be working and the role your thermals will play, and trust the insulation in your ski pants to keep you comfortable on the slopes.
Shell, or uninsulated, pants, are an excellent choice for spring skiing or if you want to select your insulation as a part of your baselayer and just want the out shell to provide weather protection. Make sure they are sturdy enough to hold up on the mountain - most rain pants are too light-duty for skiing.
Utility: Good ski pants will have details to make you look good and reduce hassle on the mountain, like metal rings or loops to hang your lift ticket, a gaiter to seal out snow around your boot, and a high back to assure that your pants don't need to be adjusted too often during the day. Some styles will have suspenders so you won't need to cinch down the waist, and tucking in your top layers is that much easier.
Durability: You're unlikely to shred your pants when wiping out and sliding down the hill. That being said, skis have sharp metal edges, ski poles have metal tips, and you probably won't be too gentle with your gear after a long day. Check to see that the seams and high-wear areas like hems, zippers, and pocket edges have been reinforced.
Mens' Fit: Freedom of movement will be crucial to good ski pants. Overall they should fit without much adjustment when worn with thermals. The inseam should will be shorter than you're accustomed to in street pants to clear you boots and bindings. The silhouette is important, but mainly so you don't catch and drag excess material. Suspenders are fairly common in men's ski pants and can be much more comfortable.
Ladies' Fit: There is a distinct difference in women's skiwear overall, but especially in pants. The silhouette is a much more important part of the design, and the pants come in two categories: Insulated and Stretch. The insulated pants have outer and inner shells with an insulating layer between and will be fitted but not too snug. Stretch pants will have a fleece lining bonded to a very stretchy shell fabric that may or may not be waterproof/breathable, and standard stretch pant fit has no 'slack' from waist to knee - no wrinkles, no bagginess.
The fabric on stretch pants is thick enough that it can create a better silhouette than conventional tights, but that's how they should fit. Because of the fit and elastic content, stretch pants cannot be as warm and waterproof as insulated pants, but offer much better range of motion, excellent moisture management, and a look that will turn heads. The main question when starting your ski pants shopping is to decide whether you want to be warm, or look hot. Decide what you want and go for it. You might need to kiss a lot of frogs to find the perfect pair, so find a shop with a good selection of brands and models. The Nils brand is notable as they make only ski clothing for women.
Deciding: In the end, if the pants have the right features and fit, you will have a better day skiing. If you look good wearing them, it'll be a great day. Don't forget that quality means the pants will last through next season or beyond, and pants you don't need to replace are cheaper than a new pair.