I have found that since there is little evaporation with crock pot cooking I need to reduce the liquids in a conventional recipe by nearly half. Otherwise my usually tasty dishes taste yucky and watery. Dense vegetables like carrots, parsnips, potatoes and rutabagas need to be thinly sliced and placed at the bottom of the pot if they are to turn out tender. Things like peas, corn, peppers and tofu or strong flavours like broccoli, cauliflower or greens shouldn't be added to the pot until the last 15-60 minutes of cooking.
At first it was hard for me to prepare a soup in the crockpot by layering the ingredients and not stirring them right away, but it really does make a difference. I put the vegetables in starting with the slowest cooking ones, then add some barley or rice, then some precooked beans, then my seasoning and finally the liquid. It usually starts out looking like there isn't enough liquid but as the water from the veggies themselves stays in the pot, there is always just the right amount by supper time.
For one of our favourite recipes, just fill about 2/3 of the crock pot with chopped potatoes, sweet potatoes, parsnips, carrots and onions. Combine 1 cup of vegetable stock, 1/4 cup of balsamic vinegar, 2 tbsp. brown sugar and a little salt and pepper. Pour it over the vegetables and cook at Low for 8-10 hours or High for 4-6 hours, stirring once every hour.