Do you mean like a summarising tool? If so then that requires an intelligent solution, plus it would need to deal with multiple languages and words that aren't spelled correctly. This isn't feasible.
Just the first 150 characters might be sufficient.
Isn't there some open-source tool somewhere that does just that (summarizes text/news articles)?
Probably, but is it necessary? Surely it would need to be precomputed and stored, altered if the topic content changes etc. Also I'm not sure how it would handle some of the lexicon on this forum - though that could be trialled. Maybe there is a learning aspect to it.
That solution would be at the higher end of the spectrum, the original suggestion is simple and will give some information that may prove to be useful at a glance.