The OP asks two different questions: (1) how to extract key words and (2) how to assign keywords a sentiment class (pos/neg/neu). I will address the keyword identification piece in this answer as many others have discussed how to do sentiment analysis (e.g., this post).
The approach I would suggest is a key keyword approach advocated by Mike Scott (author of WordSmith Tools) and Chris Tribble (a computational linguist). As discussed here, the basic approach is to create two corpora, your target corpus which is composed of texts sampled from the texts you are interested in, and a reference corpus, which is typically a much larger corpus (which is often more general in content).
The procedure begins by computing word (or n-gram) frequencies for both corpora. During this process, if a word’s frequency in the target corpus is found to be statistically probable in comparison to the reference corpus (as computed by a chi-squared test and a user defined p-value), it is considered a keyword (Baker, 2004). According to Scott (2006), the process typically identifies three types of words as key:
proper nouns, words that characterize a text’s “aboutness,” and high frequency words that are indicators of style or genre. I discuss the approach in greater length in this 2007 article where I use the method to extract the salient features of academic discourse.
To illustrate with a concrete example, imagine your are interested in identifying the key topics surrounding a target brand (say, Pepsi Cola) as expressed in social media (e.g., twitter). Create two corpora: for your target corpora you create a search for "pepsi" and for the reference corpora you could search for competitor brands of soft drinks ("coke", "mountain dew", "dr. pepper", etc,). When the keyword process has terminated you will be left with all the keywords/topics that differentiate pepsi from other soft drink brands (and as a bonus you will also identify negative keywords... words that occur statistically less frequent in the target corpus).
As you might surmise, the results you get depend upon how the reference corpus is constructed. This, in my opinion, is a feature - as it gives the researcher much greater flexibility in hypothesis testing and data exploration.