Andy I totally disagree with you. Ffrpg is totally right, it IS truly harder to hit a long, straight drive than a good hard serve. I play both sports quite a bit (5.0 in tennis, 14 handicap in golf) and can assure you that golf is WAY harder than tennis.
It's true that the ball isn't actually moving, but have you ever tried to hit a golf ball from deep rough or the sand or even on a steep slope? It can be just as intimidating as as returning a huge tennis shot.
Next you say that in golf you are not playing anybody else, which makes it inherently difficult. It's true, you aren't playing anybody else other than YOURSELF. Golf is entirely mental; you have to be able to come back from hitting a terrible shot into the water and make a birdie on the next hole.
In golf you have to be much more consistently precise; many times an entire tournament can be decided by 2 or 3 strokes out of 275 over 4 days. In tennis, look at Sherapova or Roddick and look at the dozens of unforced errors they will make in just one match.
My point is not that golf is any better than tennis or vice versa, it's just that your argument against it is simply untrue. Golf is much more technical, and if you truly want to understand how hard it is you should go try it sometime and not just base your opinion on what a tennis pro said to you...;-)