CAVEAT: These answers are assuming you're using dextrose in brewing the beer, not just at bottling to add carbonation.
1) Malt gives the beer body, dextrose does not. A 100% malt beer would have more body.
2) Dextrose is the culprit again. It ferments almost completely into alcohol. Beer is a balance between different flavors. Even if you aren't increasing the alcohol content by using dextrose (meaning you just replaced malt with dextrose), it will taste more like alcohol because it isn't balanced with the malt flavor. Look at it this way, if your "beer" was 100% dextrose and no malt, it would taste like pure alcohol. A little sweet, because the yeast would stop working before all the sugar was consumed, but very much like alcohol. Or another way of looking at it is that a shot of vodka watered down to 12 oz. tastes more like alcohol than a 12 oz. beer, even though they have the same alcohol concentration. The alcohol flavor may mellow out in time. My first beer had dextrose in it, and it tasted much less like alcohol after 6 weeks in the bottle as opposed to 3 weeks.
3) The head retention may get better with time. 3 weeks is the minimum for sugar carbonating a beer. I'd wait more before jumping to any conclusions. In the end, it may turn out that the lack of body is harming the head retention, but wait and see.
There are other possible reasons for the problems you've described, but I think the use of dextrose is by far the most likely. I used dextrose in my first two beers because it came in the kit, and I quickly abandoned using those kits once I realized what was going on. There was an immediate increase in the quality of my beer from the third time on (although I did change other things, I think using all malt helped).
If you need me, I'll be out behind the woodshed pounding Grolsches.
Primary: Maris Otter/EKG SMaSH
Bottle: Deizbeutzervet Dunkelweizen
Keg: Independence Day Strawberry Blonde
Keg: Munich/Mt. Hood SMaSH