Sentiment analysis using sources other than the IMDB data
One of the most fundamental assumptions in machine learning is that the training data is 'similar' to the test data. Training makes no sense otherwise.

So the question is: How similar are reviews of, say, movies and stocks? Maybe somewhat, but not too much.
Your movie-trained algorithm would certainly be able to deal with statements like 'This company is awesome' or 'Warning, do not buy this stock'.

But what about 'The stock price will explode/implode' or 'Concerning this stock, I am bearish/bullish'? The words explode, implode, bearish and bullish have probably never been expressions of sentiment in movie reviews.