There is a convention on the WWI forum that if you only want praise, just post the picture and description. However if you want honest appraisal from your peers, add "Comments and Critique welcome" or C&C welcome to the end of your post. It seems to work well there, and stops the "nicely turned, lovely finish" banality that often appears here.
That way, those with a fragile ego need not have it bruised, and those who seek genuine appraisal can get it, as long as they are capable of interpreting it (someone with only a little experience is likely to appraise differently to someone who has been making pens for many years) and as long as they are prepared to shoulder any criticism without tears or histrionics.
Above all, in all cases politeness and tact should still prevail however.
Personally , I rarely comment on pens here because I find it difficult to judge what people actually DO want from their posts. Many just want an audience, which is fine, some want to improve, which is fine, some want to share, which is fine, and probably other reasons too. A simple posting convention to indicate what they want would solve the problem