Super easy on the legs. I grew up playing on the stuff and didn't get too comfortable at the baseline until later in life. As long as the treads on your sneaks aren't completely worn away, the footing and movement on grass is rather good - even better if you have grass court shoes!
Clay demands that you wait before committing toward one direction or the other more than grass, but the surfaces are sort of similar in the way that they both take some speed off the ball after it bounces - good defense can really pay off. It's much easier to run down lobs on both, but the grass doesn't tee it up quite so high off the bounce - well conditioned courts are close, though.
Right after Wimbledon, there's a small pro tournament in Newport, RI that I love to attend along with the US Open at the end of the summer. The contrast is incredible because Newport's grass can be so disarming for players that depend on baseline power that they enjoy on the hardcourts. Fabrice Santoro has recently earned the trophy there and his skills (and movement!) are phenomenal to watch up close on grass. These guys need to use ferocious spin and touch to construct their points instead of using "batting practice" as a gameplan.
Grass is my favorite surface for doubles, probably because it magnifies the serve and volley aspect of the game. On hard courts, it's easier for the one up-one back formation to take hold too often, but the game is more compressed on grass and teams need more variety to press the issue. Watching the pros play doub's on the grass is routinely breathtaking... there, I said it.
