The other part to this is - everyone thinks about the total pixel count, but somehow many people miss the fact that their photographic technique may very well not be allowing them to capture even 6MP worth of detail. Not to mention that the lowest-end lenses many people are buying probably don't have the resolving power to take advantage of a high-density sensor,
Short answer: Yes photographic technique is the weak link most of the
time. No, even a cheap lens is pretty good in terms of detail.
Let's work it out.....
A 6MP sensor will have 2000 x 3000 pixels. On the Nikon this is 3000 pixels across the 24mm frame. or 3000/24 = 125 pixels per millimeter. Remember the Nyquist sampling theorem (see http://en.wikipedia.org/wiki/Nyquist–Shannon_sampling_theorem)
In real-life a 125 pixel/mm sensor can record about 50 to 40 cycles per millimeter of resolution.
This is about as good some slide film but most negative films are up to twice this good and are in the 90 to 100 cycle/mm range. We have lost about a factor two going from film to digital. Film is 2x sharper.
Nikon does not publish MTF curves for their lenses but Canon does. Let's assume Nikon and Canon lenses are about the same.... Ok back from looks at some graphs. It would seem that lenses are well matched to film.
In thise days the lens was the weak link as film is very good. Today, it seems the sensor is the week link. The best lenses are 2X more sharp than a 6MP DSLR sensor can capture. Even the cheap $90 zoom lenses can do 40 lines/mm
All that said, I have to agree with you. While technically the sensor is the weak link in the system, practically most photos will not have even 40 lines/mm of details do to technique. Some of my best images have about 80 lines/mm but they are on professional type film and shot from a tripod. Most casual snapshots are done with the camera hand help and not at the lens' optimal aperture. The D50 is a good match to this style of shooting
To get back to the quality of my old 35mm film bodies I'd need a 24MP full frame sensor. Some day digital will be there.