One of my cats wags her tail when I'm giving her scritchies, and it's pretty obvious she's enjoying the attention. It's different from the side-to-side tail lash; it's more like the motion of a windshield wiper.
Both of my cats will lay ears back and lash tails when they are play-fighting, but there is no hissing or screaming going on.
Cats don't frown with their eyes. If they are relaxed and their eyes are partly closed, or if they blink at you, those are signs of trust. I've been around cats since I was 3, and I'm still surprised when someone comments to me that they think a cat is frowning at them, but the cat in question is just sitting there with a neutral expression on its face. Next time you notice your cat looking at you, try a slowed-down blink with both eyes at her. Do this enough and your cat will realize that you're using her language, and may start doing it back. Likewise, if your cat blinks at you, blink back.
Look also at whisker position when engaged with your cat - forward means interested, far back is defensive. But if you are scratching a cat on one side of its head, it may pull the whiskers back on that side; that's normal.
If you are petting your cat, look for signs of relaxation like the 'sleepy' eyes. If your cat doesn't seem to enjoy what you're doing, try something different or give the cat a break. Some cats don't enjoy petting on their torsos, but only on their head, back of the neck, and ears.
If your cat bats at you, she may be trying to play-fight, she may be trying to chew on your fingers (one of mine does this as a sign of affection, I think it's because when she was teething as a kitten I was the only thing she wanted to chew on, and the habit stuck) or she may be signaling 'enough.'
Cats live 'in the now' and don't stay angry at people or circumstances after the negative stimulus stops. They can, however, exhibit conditioned responses, just like many animals.