Freezing damages cell walls, leaving limp vegetables. Most greens are chopped, which works in some recipes. Always use fresh greens, except for spinach.
High-water-content vegetables, such as celery, do not freeze well for similar reasons as leafy greens.
Compare soggy, flavorless frozen chopped onions to fresh ones. Defrosting melts crystals, leaving behind ice damage.
Cruciferous veggies don't like the freezer because they're so watery. Thawed, frozen vegetables are limp and lack fresh flavor.
The melting ice crystals will harm the sensitive leaves of Brussels sprouts if they are thawed prior to cooking.
Pasta freezes well, especially when paired with the sauce. But, if frozen pasta wasn't al dente, it may be mushier than you want.
When frozen and thawed, dairy-based dressings and sauces separate. The outcome looks awful and tastes bad.
Pectin is used to thicken jams and other foods. The way pectin gels changes when it is frozen, resulting in liquid but lumpy jam after freezing.
Don't freeze uncooked tomatoes. When tomatoes reach 41 degrees, a molecular shift turns off key flavor components.