Flip-out screens can be useful for tricky shots.
We consider a lower weight better because lighter devices are more comfortable to carry. A lower weight is also an advantage for home appliances, as it makes transportation easier, and for many other types of products.
With 100% coverage, you can compose the image correctly when you capture the photo. With less than full coverage, you may have to crop your photos afterward to get them looking perfect.
An optical viewfinder (or OVF) allows the photographer to compose a shot while seeing exactly what the lens will capture. OVFs have no time lag and require no power - as opposed to electronic viewfinders, which can drain battery life. OVFs are also better in low light situations.
A hot shoe can be used to attach an external flash, as well as light meters, viewfinders, rangefinders and other attachments.
A higher screen resolution provides a sharper image, making it easier to review your photos.
A system camera has many interchangeable components such as lenses, flashes, and other accessories. System cameras offer greater flexibility than compact cameras.
A flash is good for low-light environments, and can sometimes be used as flashlight.
The bigger the screen size is, the better the user experience.