When I started playing golf, I was hoping that learning to play golf would be relatively easy for me, because I used to be a pretty good tennis-player in my younger years, and I was hoping that my tennis experience would help me to master the golfswing quickly....

.... I found out that I was wrong!

After years of golf-practice, I have finally developed a decent golf-swing, and I am now as comfortable with a golf-club as I once was with a tennis-racket (I no longer play tennis). And I think that I have figured out why my initial assumption, that a tennis-player can easily learn a golf-swing, was wrong, and I will try to describe why: I will come straight to the point:

A right-hander's golfswing = A left-hander's tennis backhand.

What I am saying here, is that a correctly executed golf-swing by a right hander has a lot of similarity to a correctly executed (double-handed) tennis backhand by a left-hander.

However, when a right-handed tennis player attempts to hit a golf-ball (setting up like a right-handed golfer) he would be tempted to apply his right-side dominated forehand motion to the golf swing, with bad results.

On the other hand, if the right-handed tennis player wished to take his tennis-experience with him to the golf practice range, he might be better off setting up with a left-handed golf club, and hit his familiar (double-handed) backhand, which is a much better motion for golf.

No wonder that golf is so difficult to master: In tennis, the backhand is much harder to learn than the forehand (at least for the typical player), but the golfer is required to learn this more difficult (backhand) motion opposite their natural inclination!!

To expand on the previous paragraph, it is interesting to note that a number of golf-legends (Johnny Miller, Phil Mickelson, Ben Hogan, Bobby Jones) all play(ed) opposite to their natural inclination. While it may feel that this is a more difficult method to learn to play golf, it may actually be easier in the long run. I say this for the following reason: While the tennis-backhand is more difficult to master than the forehand (for most), it actually is a more natural feeling stroke once mastered. The players mentioned may have actually had an advantage!

The bottom line:
If you are left-handed, do not hesitate to play golf right-handed.
If you are right-handed, do not hesitate to play golf left-handed.

I recognize that not many people would be willing to switch over, and retrain themselves for a mirror-image golfswing. However, if you are a beginner there may be some advantages to playing golf opposite to your natural inclination ('handedness'). If you are not progressing, or have a deeply ingrained horrible golfswing, I would recommend that you give yourself a second chance by starting over playing the other way. Do not expect quick results however: You will have to learn the golfswing again, but this time you may be able to avoid some of the mistakes you made in your first attempt!
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PS I realize that this post, long-winded as it is, is missing something, i.e. a justification for the statement that a correctly executed golfswing resembles a backhand motion, and not a forehand motion. I will leave that one alone, for now, but there are hints in the following thread (posted by Divot2Deep), which is very short, but very interesting:

Any thoughts on my 'tweaking'?

I have wanted to post these thoughts for a long time now, but it is the above thread that prompted me to finally do so.