Under the hood they have a new DX sensor which they have rolled out across the entire DX range (bar the 300s). The sensor or possibly processor gets crippled in the lower models eg 3000/5000 range, so the get the same resolution but features / ISO performance / fps isn’t quite as good.
In the D7000 you get 6 fps, super auto-focus (51 point, 3D tracking and colour tracking). As for does it take better pictures than a D70 / D80 (or other body) – it depends on what you’re doing. For landscape – no noticeable difference. For sport / action, the faster auto-focus and 6 fps blow away the D70/80 and make a huge difference. Low light performance (high ISO) is also much better than the older sensor – but no where near as good as the FX bodies.
Whether you want to upgrade depends on whether the features make a difference to you, as someone into sports photography it was a no-brainer, just for the increase in fps and better high-ISO performance. I don’t give a monkeys about resolution – 5mps is quite enough for most photography.
I’ll be buying the D4 as soon as I can pre-order one, just for higher fps and even better high ISO performance.