There is a lot of functionality crammed into Drupal. This can be a good and bad thing. One of the key benefits is that its components are well maintained. You have hundreds of smart people looking at the code every day and thinking of better and smarter ways of implementing the functionality. One of the drawbacks, however, is that it has a long release cycle, dependent on when a new version of Drupal is released. This means that if there are any new features put into that module or theme, it has to wait a year before it gets out to the general public in a new Drupal release.
The following is a list of issues created to strip some of that functionality out of core:
The access rules implements some pretty complex access control on a user/ip basis. Although it's a great feature, the way it's implemented requires a call to a function that is the slowest query on drupal.org. Moving this to a seperate contributed module would allow some more added functionality to the access control, as well as improve Drupal usability.
The commenting control within Drupal is very complex and doesn't really help with usability. It would be a better idea to open up the API, and allow hooks to allow external modules and the theming layer to implement the controls.
There is a little button that appears beside feed items named "Blog It", which allows users to stick it on their own blog on the same site. There are millions of reasons why this shouldn't live in Drupal core, so it will be good to see it go.
This is just the beginning of stripping Drupal to a really small, slick, and fast framework. There are a lot more issues out there, and a lot more things that we could/should take out of Drupal core, so get involved and give your thoughts!