If you have never played on clay, it's not easy to understand the strength that it takes to play on it and the massive difference between Clay, Hard and Grass. I have had the 'pleasure' of playing on Clay, but don't know that I like playing on it. Here's some background on the Clay court.
Clay courts are made of crushed shale, stone or brick. The French Open is the only tournament of the grand slams that is played on clay (also deemed the toughest); the others are played on grass and hard courts.
Clay courts slow down the ball and produce a high bounce in comparison to grass or hard courts. This is because clay courts have more grab and when the ball lands there is more friction pushing against the ball's horizontal path, therefore slowing it and creating a higher bounce. This type of court court takes away some advantage of big serves and 'winners' which makes it hard for serve-based players to dominate on the surface. Rallies last quite a bit longer as we saw with some of the points in Serena's first French Open match this year. You also have to learn how to slide on clay, some players move better on this type of court and have an advantage.
Clay courts are more traditional and cheaper to construct than other types of tennis courts, but the maintenance costs of a clay surface are higher as well. Clay courts need to be rolled to preserve flatness. The clay's water content must be balanced. Clay courts are more common in Europe and South America than in North America and tend to heavily favor baseline players.
There you have it; clay courts create a whole new bounce, speed and spin. For a player that plays really well on a grass court, clay may take some time to adjust.