The question in question is this:
Which feature in C/C++ don't you like?
Just wanted to know if that is proper way of asking. Not sure if "don't you like" is the right way there.
If your intent is to ask for the reader to respond with a single feature of C++ he or she dislikes, I would phrase it:
It's more common to use which of or what of rather in.
As for which vs what, the Chicago Manual of Style says:
But I believe which is correct in this case because CMS also says:
As TimLymington notes, your question is phrased in an informal way. If that's your intent, that's perfectly fine.
Yes, it's correct grammar. Don't is informal, so not suitable for, e.g., a manufacturer's survey; it's unusual to say 'feature' (either 'features' or 'single feature' to emphasize what you want); and a pedant might point out that you don't actually like all the features you've never thought about, so dislike would be better. But these are points of style, and much depends on the context.