I know sometimes people don't take well to criticism.
For someone to point out all the negatives aspects of ones argument with the intention to deflate the whole side of the argument - i would regard as bashing. I often see people do this when quoting an argument and commenting on each part. [edit] There's also the normal kind of bashing where someone resorts to commenting on your personal nature to get their point across. This is definitely bashing and unacceptable to resort to such levels to 'win' an argument.
Criticism i would regard as pointing out simple flaws in the idea rather than the whole idea. You can say the idea can work unless you change this or that rather than this and that is wrong with it - it just won't work! In such things an open-mind is required.
These are just my views on criticism and bashing.