Durability of the fabric, poles, waterproofing and zippers to start with. Cheap tents will often have a fly that doesn't reach more than halfway to the ground, so you're dry as long as you don't have rain and wind together. Better tents will have more interior pockets, pockets that are attached more securely, and maybe attachment points for a gear loft. They'll also have better ventilation for a given level of foul weather resistance. Other features might include a second entrance or an integral vestibule to store gear.

If you want to save money, consider an A-frame tent like the Eureka Timberline. The A-frame construction is much simpler and cheaper to make than a dome tent, so you get features like real aluminum poles and sturdy fabric that you wouldn't find on a dome tent for a similar price. The downside is, A-frame tents aren't as roomy inside, and tend to weigh more for a given amount of interior space.