There are some things that other pieces of software do really really well. When you can borrow or copy their wheel, why go to the trouble of building your own from scratch?
The first issue that needs to be addressed is anything having to do with the installation process. People won't give it a second chance if they can't get it work "out of the box". The super-clean, super-simple, almost always flawless install is a major reason WordPress has the market share it does.
For example, I ran into an issue when trying to install Drupal 7 where the database didn't get fully created so the install failed and flatly refused to continue for no apparent reason. It took me a few hours of googling to figure out that's what the issue was. It's a few extra lines of code to make sure that all the tables are there and that all the columns for them are there too. Otherwise, drop the database and try again.
Second, if you feel the need to add core functionality, look at the "flagship" products in that category. If you want to see how forum software should function, look at SMF. Since they are now open source, you can probably fork their code for use as a forums module in Drupal. If you want to see how a blog should function, look at WordPress.
Third, I find that there are some specific features in other products that Drupal might want to emulate within Drupal. Copy what's good but improve upon it when doing so. For example, installing WordPress MU is about 1000x simpler than installing Drupal as a multisite, largely because it only requires one flag in the config file. The rest of the process happens through the GUI Admin panel.
Another example is the module & theme install process in WordPress. What makes it more functional than Drupal's is that I am not required to paste in URL's or manually upload files. It searches the Wordpress repository. This is something that Drupal could do easily.
Where it falls short is that I cannot define additional custom repositories. For example, I would like have one in house for my own custom modules. I might also want another (or more) from vendors who are supplying me with modules. This would allow me to publish or receive updates from all of my repositories at the same time.
Fourth, I find that there are items that are simply lacking. A shared media library with the ability to limit upload rights. I don't know about your users but mine think absolutely nothing of uploading 64MB 1080p landscape still-frame and trying to shove it into a 150 x 300 pixel portrait photo slot.
Fifth, I need to be able to insulate my users from having to learn a lot of jargon like node, taxonomy, blocks, and entities. Imagine trying to explain all this to your grandmother. Now multiply that by 300... Now you see why I say I need to be able to insulate my users from that.