There are too many emotions that are considered negative, but probably one of the most vilified is anger. I really like the work of Karla McLaren on emotions, she calls separating emotions into positive and negative lists: 'valencing'. You can read some excellent articles on her site on anger and all of the other emotions. For example she has a list of the gifts that anger has for us which are:
Identifying the gifts of anger
I have a series of questions for you, and I want you to think about them in the present moment — in relation to your current relationships and your present-day skills. These answers can change over time, so let’s focus on your situation right now.
As you answer these ten questions, please grade your responses from 1 to 5: 1 (No – Never); 2 (Rarely); 3 (Sometimes); 4 (Often); and 5 (Yes – Always).
I feel heard and respected in my interpersonal relationships
I am comfortable speaking up for myself, even during conflicts
I take good care of myself
I know who I am
I can make clear distinctions between my own needs and the needs of others
I can say no to demands on my personal time
I can make clear distinctions between my own emotions and the emotions of others
I can remain present and focused when others are angry
I am sensitive to issues of social justice
I work to make the world a more just and loving place for everyone
It's easy to see how anger can serve us after reading this list. The problem is most of us either repress anger or express it when it's gotten to the point of rage because we've let it build up. The trick is to acknowledge your emotions AND any judgements you may have for having them in the first place. The most important thing is to feel your emotions so they don't get stuck, you don't necessarily have to express them if you don't want to or don't feel safe in doing so.
Personally, I would never work with a practitioner who spoke about emotions being either positive or negative as I'd find it very hard, if not impossible, to be emotionally honest with that person, especially if I were feeling any of the 'negative' emotions on their list, which would make any work we'd do together futile. Why is being honest about how we truly feel such an issue with ourselves and others? Something definitely worth tapping on ...