Are you talking about a recommendation or a comment?

I'd put concrit into a comment only when asked. A private message is a good option too. Sometimes if I really love a story but there is something that bugs me about it, I put it into my review too - among other things and keeping in mind that a lot of these things are a matter of taste.

When recommending a story elsewhere (also if the writer can see my rec), I point out all the reasons to read it as well as my personal warnings like OOC or poor grammar. Some people won't mind a bit of OOC, others will rush to read the story since it's SO good despite its shortcomings... You provide information and people decide what to do with it.

I think you're fine either way as long as you come across as a friendly person sharing your opinion.

Well I knew what you meant and my point stands the same!
Just make your comment as accurate as you can, you are not being rude- just giving your honest opinion.
For example: on the one hand I would recommend this fic because I enjoyed the story, however it could possibly have been tidier in its presentation...something like that.
 

Yeah, I agree that you should be more careful in a review, and should always point out the positive things about a story,not just criticism. However, I wasn't sure if it was ok to mention issued with the fic in a rec.

I have just caught up on this conversation, and I have to agree that if it's what your writing in a recommendation to someone else who is looking to read it, then it's OK to mention both what you liked and what you didn't like.
As for comments, I never thought there was an "unwritten rule" about not giving constructive criticism. It's OK to say something like "I really like your story, especially the part where character X does thing Y. I thought the dialogue seemed a bit stiff at times, but overall I liked it." Something like that is still overall positive, I think, and I don't think a writer would be hurt by it. Just commenting "I hated this, you suck," is just mean and not useful at all, but that's not what you're thinking of doing, so I don't think you should worry about talking about what you think is a story's weakness if you still liked it overall.