Plant cytology

Each tissue and region is composed of various differentiated cell types which together provide for the functions achieved in the tissues. Several different cell types can be observed in plants.
There are wax-coated dermal cells, isodiametric parenchyma and elongate collenchyma ground tissue cells. Some of the latter are sclerenchyma cells. Each of these have a range of structural and functional features that distinguish them .
The hollow tracheary elements of xylem are most closely related to sclerenchyma in structure and function. The living sieve tube elements are most closely related to parenchyma in structure and function, but are clearly more derived. The sieve tube elements typically lose their membrane-bound organelles including nucleus, mitochondria, and plastids. An adjacent complete parenchyma-type cell, called the companion cell keeps the cytoplasm of the sieve tube element alive through plasmodesma connections.

What is it that all plant cells have in common?

To be honest, perhaps there is nothing. Some cells lack cell walls (plant sperm cells, for example) and other cells lack everything else (xylem vessel element, for example). But because of cells that are dead at functional maturity, the best answer would probably be the cell wall. In consideration of the sheer mass and volume of dead cells in the trunk of a Sequoia tree, for example, a dead cell might arguably be more "typical" than what is often depicted as a "typical" plant cell in textbooks.

Because a parenchyma cell is alive and is responsible for virtually all of the photosynthesis and respiration in a plant, most books will show the parenchyma cell as "typical." Below is a cartoon of what a parenchyma cell looks like in an electron microscope view. This cell could be, for example, a spongy mesophyll cell from a leaf.

The cell wall is more than just cellulose

Many people know that the cell wall is made of layers of variously arranged/aligned cellulose microfibrils. But this polymer of glucose is not the only wall element by any means! In addition to cellulose, walls have a range of various polymers of sugars and sugar-derivatives. Hemicellulose rhamnogalacturonan, and pectins are shown here.