What features make one enlarging easel better than another?

Simply put, just like the there is a difference between a Volkswagen Beetle and a Porsche 911, they both get you from point A to B, there is difference between basic and advanced.

One is more enjoyable and efficient. Enlarging easels are more than just a board, they have a fixed or adjustable boarder for cropping your photo (cropping frame). This also serves to hold the paper flat to the board which is very important to maintain an in focus image on the paper. Some have clips that hold the paper in place and down while the cropping blades are up (many photo papers have quite a curl to them).

The Porsches of the boards have four independently adjustable blades with large adjustment handles for smooth action and a graduated ruler so you know where you have set it for a particular negative (if you take notes, and you should). Some have only two blades that adjust with the other two sides fixed. The blades or cropping frame is hinged and the better ones will have indents so it will stay up on its own while you are positioning your paper into place. For this reason the board should have substantial weight and sticky rubber feet so that it stays in place and does not move as you lift the frame or make adjustment to the blades.

Basic ones like the one I got in high school have fixed (non-adjustable cropping) frames, it had an 8x10 on one side and on the other three openings, 5x7, 4x5 and 2x3 if remember correctly. It is a very functional piece and is quite useful still.

An even more basic one is just a piece of thin steel with grooves or tabs or slots on the side and you slide the paper in under the tabs, no moving parts.

While technically you don't need one to print a photo (you could walk form A to B) it makes the experience more enjoyable and efficient. And importantly it gives you the ability to reproduce the exact same crop every time you print the same negative.

There are also ones that are specifically made for making a contact sheet of your negatives (also from basic to deluxe). I just use a piece of glass and the base of my enlarger.