There are also possible extensions to the lexical/morphological features -either in the direction of semi-supervised learning and adding for example features based on latent semantics, topic models, or graphical models popular in the sentiment analysis field , or the direction of deeper linguistic processing in terms of, e.g., syntax/dependecy parsing (but this has limitation given the nature of Twitter data as well as unavailability of such tools for Czech).
we approached the problem mainly from the data-driven perspective (annotation, feature engineering, error analysis).