Nikon would like it if you upgrade each time they come out with a newer camera model. That is why they keep adding more features.
Necessary ... No
If you have mastered all of the features on the D3000 you could upgrade just for the challenge of new features.
It would be rare that a newer DSLR will produce better image quality.
Nikon wants each model to produce the best quality photo possible.
They never dumb down quality for a lower cost DSLR.
A newer model is all about more features.
Here is a comparison of specifications:
Here is a full review of the D5000.
Read it and you will learn all the new bells and whistles.
A good selection of lenses is important.
I've had the D3000 for about a month and I'm quite happy with some great photos it took. However, I'm curious about the D5000, and I can afford it right now if I sell/trade the D3000. I'm only concerned about image quality, not any other features. I hear the processor is the same as the D90 and D300, which should make some great shots right? Or is it just dumbed down and I really won't notice the difference between the D3000 and D5000? I have about 10 years photography experience and only ever worked with low end DSLRs (budget), but I do own some nice glass. I'll be using the camera for travel mostly. My only concern is the difference in image output quality between these two cameras and if it's worth getting the D5000 instead.