Drying out: plants need to conserve water since they live on land.
Gas exchange: a different means of exchanging oxygen and carbon dioxide that diffusion across a membrane had to be evolved
Support: Water supported the weight of the algae, plants have evolved a cellulose structure of cell wall to support their weight.
Reproduction: gametes released into the water could mix and mingle swimming with flagella until they meet causing fertilization to occur. On land, reproduction had to evolve to include wind, animals, and other means of getting the male gametes to meet the female gametes.
There are more, please follow the links below for more details. These are the biggest obstacles to life on land that a plant will face.