The main purpose of having a power rack is safety. Thus it is important to choose a rack with high weight rating. These values usually represent the static weight load, so a safe gauge will be to have the rating double of what you can lift.
Narrow hole spacing is required for setting the safety bar at the correct height. For example when doing bench press, if the safety bar is too high, it will reduce the range of motion when benching, and if it is too low, it is the same as not having it. Recommended hole spacing is the most 2 inch apart, and of course smaller will be better.
The rack must be wide enough to allow enough room for workout inside. However it cannot be too wide that when re-racking, you need to be precise to make sure the inner bar fit in the j-hooks. Another problem with a rack too wide is that the plates will hit the upright while you are moving the bar towards / away from the j-hooks.
Ability to accommodate your bench
Make sure that your bench can go deep into the rack for bench press. Some racks comes with connecting bar that will block the bench from going to the required depth.
Chin up bar
Other than doing chin up, you can also hang bands or suspension trainer for variety in workouts. Some racks come with multi grip chin up bar which is a plus.
Without padding, the bar will be in contact directly with the metal part of the j-hook and this will damage the knurling of the barbell. Additionally, it is also noisier.
These features are not a requirement but good to have.
For cable exercise such as lat pulldown, low row, tricep etc.
Needs to be height and width adjustable to fit requirements of different user.
For band assisted / negated workout
Weight plates holders
Keeps your home gym neat as well as makes it convenient for you to load plates to the bar as the storage are usually very near the barbell.