Academic writing is rational, and if you appear to be persuading your reader by making an appeal to their emotions rather than their reason, this may lessen the impact of your work. Emotive language to avoid might include:
Implied value judgements (some value judgements might be appropriate, depending on the question you are answering, but they need to be precisely stated and backed up by reasoning and evidence. Other value judgements may imply a lack of objectivity). Value judgements might be implied in a choice of word which is not neutral but has positive or negative connotations.
Intensifiers such as really, very etc. They do not carry any precise meaning, and therefore do not add any value to your argument, but are an appeal to your reader's emotional response. The use of italics is sometimes also used to intensify meaning, with the same implications.
Assumptions about the shared agreement between you and your reader. Using expressions such as clearly, obviously, of course imply that your reader already agrees with you, when this agreement must first be reached (if at all) through reasoning and evidence. Once you have stated your case, then these words may seem redundant - your conclusions are clear to your reader or they are not.
Rhetorical questions are posed for effect, and are not answered by the author; the answer is meant to be supplied by the reader. They imply that the answer is obvious, and that writer and the reader share a common understanding without having to construct a chain of argument. The answer may not in fact be obvious or even uncontested, but even if it is, you need to demonstrate your thinking so that it can be tested by your reader. Raising questions is a good way of progressing your argument, but you should always answer your own question.
Humour is rarely used in academic writing and is used sparingly and with caution. Humour can be used effectively in students' academic writing, as it may underline your appreciation of a point and enliven your writing. However, for some readers, it may undermine the formal impression you are trying to create. Humour does not add anything substantial to your actual argument; entertainment is not the primary aim of academic writing, and this should never obscure or eclipse the writer's point. Humour is an emotive rhetorical strategy which aims to promote a feeling of complicity between writer and reader on the basis of a shared understanding of a joke, whereas an academic writer primarily uses logical argument and evidence to persuade the reader. You may find it occasionally in the work of very established scholars, but to your reader, it may appear very differently in that of a student, so use it with care.
Exclamation marks are usually inappropriate in academic writing, as they are used to indicate an emotional response of surprise or excitement.