In some files (e.g. in text files using newline as a paragraph delimiter) one can get matches in very long lines that can occupy several screenfuls. These are not always straightforward to avoid. There's no way to tell (GNU) grep to truncate its output; perhaps Emacs could help here, by being able to specify a maximum length of match to show? For bonus points, show part of the line that matches, with ellipses, rather than just truncating the line; but even without that it would be useful, provided that the truncation is clearly marked: the annoyance and slowness of navigating *grep* buffers full of enormous matches which are almost always not what one is looking for would be great!
(I just managed to work around one source, minified _javascript_ mixed in with non-minified files, by omitting *.min.js in the list of files grep should ignore, but this is not always easy either: sometimes one might only have the minified form to hand, and might want to grep it e.g. while debugging.)