I call the following annoyances rather than bugs because yum is working as designed. What I really wondering is if an enhancement to yum is appropriate. While it may be appropriate, these cases may also be in the "too hard" or "not worth the effort" category.
I will also caveat this by saying that I have only seen these cases working with the development tree and not with "production" version. I will also say that a "fresh install" will not have the problem but, even in development, I prefer updating rather than installing unless I am testing the installer.
Case 1: A package is install but, now, has been deleted from the development/update tree and the installed package blocks updates. The current answer is to manually remove the package (yum remove or rpm -e) if, and only if, you know that package was deleted .. the recent case of dbus-qt.
Would it be possible to have some metadata which would flag to yum that the package needs to be removed.
Case 2: This is similar to case 1. In this case, an older version of a package is in the development/update tree, The currently installed packages include one or more packages which are a "more recent version" but one that has problems ... therefore, we need to "roll back" the package. Again, this can be handled manually via yum remove/install or using rpm --oldpackage if you know that the situation exists.