Based on my personal experience, having separate tags for feature requests and bug reports leads to occasional bickering about what counts as which, distracting from discussion of the actually important parts of them.
On the other hand, distinguishing between what is and isn't a problem is important—after all, if something doesn't fix a problem, then it won't end up being used and will just end up bloating things. Having a single tag for issues would encourage insightful discussions about whether something is really a problem.
I've personally found that feature requests tend to be issue reports in disguise, even if the reporter doesn't realize it—usually, it's completely possible to reword them to show how they would fix some sort of problem. If that's not possible…well, it's not really worth implementing, is it?