ultravox, I can sympathize with you. I thought of myself as a pretty good photographer with film and loved my Canon AE-1, but when it developed crack in the seal I decided to go digital instead of buying a new film camera. There are a lot of things I like about digital, but there were many new things I had to learn and the camera does not work the same as a film camera when you're shooting manually.
I shoot a lot of sports inside and out, like right now it's a lot of night football and volleyball. When I first got the camera I took it to a basketball game and shot a bunch of what I thought would be good photos, but when I got home and looked at them they were all covered in a red haze. I didn't know anything about white balance either. A call to the camera shop helped me realize that I had to adjust the white balance for the yellow-reddish tint given off by the lighting in the gym. No problems after that. Now I've gotten the manual settings figured out well enough to adjust for it without even having to use white balance, I can just use the flash setting and still have them turn out fine.
Those photos were also almost all blurred. I had set the shutter on 1/60th like with a film camera and flash, but clearly that wasn't correct. I finally got used to the digital camera's higher sensitivity to distance, flash settings, etc. If you're going to use it to shoot action stuff in low lighting conditions you'll have to vary the settings quite a bit depending on the shot you want. Volleyball in the gym is usually good though at 1/80th, 4.0 to 5.0, and flash at one-half to one-fourth power depending on the distance. The best general setting for night football varies greatly depending where they are on the field, but with a 75-300mm lens a good general setting is 1/160th, at TV priority, flash at full power, and exposure compensation at + 1 and 1/3 to + 1 and 2/3rd for middle of the field shots and the same settings and no exposure compensation for closer to the sidelines. Though 1/80th with no exposure compensation up to +1 is better for very close to he sideline stuff.
I'm still getting used to the center-weighted and evaluative metering though. When your're trying to get a shot of something on the field or court it can make a big difference in what comes out in focus. I usually use center-weighted because other people on the field or court can get in the way and cause problems when you use evaluative, but it's a problem at times keeping the center point exactly on the action you want in focus, it can be really picky. Like Friday night I thought I had a perfect photo of an interception at a football game, but when I got home and looked at it the two guys involved were not in perfect focus. Though I had the center spot on one of the guys, enough of it was just below his arm to make the camera focus on the linemen in the far background where the ball was thrown from and that's what was in focus.
I was also having problems shooting running backs coming right at me and getting them in focus. After all, with the old Canon those were the easiest to get in focus as opposed to the play running by you. It took a while before I finally read somewhere that the automatic focus on most of the lenses had a hard time keeping focus on something coming right toward them. Once I found that out I figured out that all I have to do is keep the camera on them and hold the shutter halfway down longer to give it time to adjust and they come out fine now. Of course it gives me less time to get out of the way, but those are the risks you take to get a good picture.
Exposure compensation is fun to play around with too, though I find that it tends to soften some photos. Having the ability to edit your photos just the way you want them in Adobe Photoshop is a lot of fun too, though having all the choices of ways to crop them can drive you crazy sometimes figuring out exactly what looks best.
Just get out there and play around with the settings with a variety of stuff before you use it for those important photos and you'll be more comfortable with it after a while. Good luck!