A few things.
On a reflex (aka SLR), all those modes (Night portrait, landscape, Macro) are pretty much useless. On a point and shoot, the macro mode allows the lens to actually focus much closer by "physically unlocking" the lens.
On an SLR, the lens is the same, and if it doesn't have a real macro mode, it doesn't have it. Period. The infamous "macro" setting is just a comercial feature to prevent you to say, if it wasn't there "Hey, how is that the D40 doesn't have a macro mode and the Canon rebel has it? I'll take the Canon".
I suspect it has another purpose. In every picture you take in program mode, the camera takes in account a lot of factors.
For instance, it will measure the light in different areas of the frame. If, to take a simple example, it detects that 1. the upper part is very light, 2. the bottom is dark. 3. In the center it's dark too, it'll compare this to its internal database, and probably come up with a result like this:
The data are the same that in the reference picture number 34398,
ergo this is probably a portrait of somebody (the dark center of the frame) in front of a landscape, because the sky is light (the upper part of the picture) and the ground is darker (the bottom).
In this equation (comparing the image in front of the lens to its internal database) it takes in account the subject distance too. For instance, in the previous example, it would have seen that the center of the frame (the people) was 9 feet away, and the upper portion was at infinity (the sky).
WARNING! DON'T READ THIS IF YOU'RE -VERY- EASILY OFFENDED!
With the idiot modes, you are only giving a tip to the camera even though in P it works generally fine, she's "smarter" or "savvier" than the average user 99% of the times. With the macro, you might be overriding the camera-subject distance... But again, she's already smart enough to know it!
On the other hand, those modes just give the user the feeling he's doing something, and he hasn't paid 600 $ for a point and shoot camera.
It's the same thing with the manual mode. Often, people just set the mode dial on M to feel like they've chosen the exposure. Alas, when they do so, it's often only to make the exposure arrows match the 0
+ l l l 0 l l l -
_<-----l
"How, that's overexposed!"
*turns the dials*
+ l l l 0 l l l -
______l
"Now, that's better, let's press the shutter!"
Sadly, the meter on the camera is just saying what the camera thinks is the correct exposure (what she'd have set if it was in P mode) so by doing this you are only manually turning the dials to get exactly what the camera'd have done automatically, but she's still deciding!
The correct way to use the manual exposure is looking at your scene. Deciding which part you want to be well exposed (or not, you may want to overexpose intentionally), set the spot or, if you don't have it the center metering. The smaller is the metering area, the better you can narrow the area you want to expose correctly. Once you've decided the area you want well exposed, you point exactly at it with the spot metering, and you turn the dials to get the 0 mark. Now, you can reframe, and press the shutter with total confidence of having done it yourself.
An alternative (not he only one) would be setting the P mode, the spot metering, pointing where you want, pressing the AE-L button (locks the exposure), reframing, and shooting.
Now, have fun.
