I can assure you the d5000 will give you the performance you need as an upgrade from a point and shoot.
Condensed, slr's generally offer detachable lenses for more versatility (if you need more zoom, buy a zoom lens. If you need wider angles, buy a wider angle lens) and you can find a lot of lenses for 300 and under, although they'll have slight disadvantages from spendin a considerable amount more, but depending what you're shooting, and budget, it may be well worth an upgrade. I would ask advice from a local camera shop and forums, but don't believe everything you hear. Some people are so biased they are sure opposing brands are awful, which is hardly the case. My friend owns a d5000 and I've seen some breathtaking photos from it.
The reason why they're more expensive? Bigger body, the versatility in lenses thing, speed of focusing, speed at which you can take one picture than another after that (d5000 has a 4 frame per second continuous shooting mode, meaning if you wanted to, you could take 4 pictures in one second for up to 20 seconds or so!), and size of the sensor. If you're familiar with film, look at it this way. The sensor is like the film of digital age. In smaller cheaper cameras, they can advertise "15 MEGAPIXELS WOW", but it's al being captured by a sensor that can be the size of a fingernail. When you buy an SLR, you're almost guaranteed one about the size of a....cheez-it, for lack of a better comparison. The reason why megapixels don't matter to much, is let's say you take a picture with a 15 megapixel cheap camera. That means it has fifteen million colored dots that create your picture. If you were to use a film camera and try to capture all that information on a teeny tiny piece of film (compared to the 35mm rolls you might be used to), and you blew it up to a big print, it would probably be smudgy and not very crisp, the film isn't large enough to capture all the colors and lighting, so you get a lesser quality picture.
Now enlarge that film to it's regular size, you can get some amazing pictures due to the fact it is large enough to capture the image in full quality. Given most digital SLR sensors aren't as big as a 35mm negative (unless you have some money), nearly any new slr on the market will give you similar, if not better resolution and quality.
I hope that was more helpful than confusing, I just kind of spewed it out