Here are some things that are better not frozen.

Salad greens and raw veggies that you intend to use in salads. Freezing them makes them lose their crispness.

Eggs in their shells - they expand and crack.

Hard-cooked egg whites - they will become rubbery.

Potatoes - Only mashed potatoes freeze well. Raw potatoes become mushy when frozen. If boiled first, they become watery and tough.

Stuffed Poultry - It can develop bacteria. Freeze poultry unstuffed.

Flour and cornstarch mixtures - Thickened mixtures  tend to breakdown and seperate (get watery)

Crunchy foods - They lose their crispness.

Yogurt, sour cream & mayonnaise - Freeze these items only if they are (or will be) used as part of a cooked casserole since they seperate when frozen.