Why is it that megapixels don't really matter and why is the sensor more important? I've always wanted a camera
Part of it is a mechanical issue of, "angle of acceptance". As you decrease the size of the individual pixel, the light must be closer and closer to a true right angle to enter it.
This is an effect you could interpret in a monitor or TV also. As the size of a TN panel decreases, the acceptable viewing angle decreases, and IMO, fairly dramatically. (Other panels obviously suffer this effect as well). But, you can view a 40" TV from just about any angle and get the same result. (Different panel tech, plus way more pixel size).
The second issue is noise, and the S/N ratio tends to get better as the pixel size increases. The inverse is true, in that it requires better tech and construction, to get rid of the noise in smaller sensor units. Likely improvement in the firmware as well.
A present there is a market upsurge in "full frame DSLR" offerings. "Full frame" is considered to be having a sensor the same size as a 35mm negative, or 24 X 36mm. These are what have been considered "pro cameras" for years.
Ironically, "digital lenses" have been developed to compensate for the smaller sensors and light entry angle. ("Digital lenses", if designed properly, bend the output to the sensor back to straight, after the image production has been completed). Unfortunately, these lenses are designed for smaller sensors, and their, "coverage circle" falls far short of the 35mm frame size. Now, the pendulum will swing back towards to the older format 35mm coverage circle designs. I suppose the moral here is, I hope everybody held onto their old 35 mm lenses, you're liable to be needing them.
