I own the new Naga and an original 2009/2010 era Naga. Differences
1. New Naga feels cheaper, a lighter and cheaper feeling (maybe not cheaper but a rougher more matte feel) plastic without the ruberised coating you usually have with Razer mice. They must have been thinking cost saving to switch from their signature coating/feel.
2. The travel on the click is shorter, this actually feels better to me, more precise and to the point, an improvement. Shape wise your fingers lay on the top of a fairly flat surface on the top of the click, instead of the old design where they sit in a finger shaped groove.
3. Size wise the new Naga is bigger, it's wider and though it isn't longer there is less of a curve to the top, so the high point at the back of the mouse is further back and then it is flatter along the mouse, so your whole hand will sit on it more, less encouraging to claw grip compared to the original Naga. The side grip is closer to the "fat/wide" grip of the Naga epic, and no changeable sides so you can't revert to the old style side. The side also has a rubber grip that works nicely where your little finger will sit.
4. Movement wise it feels the same, though it has the 8200dpi laser and slightly differently placed slide pads at the bottom (there is no ring around the whole thing but rather 3 pads, 1 at each corner at the front and another that covers the whole heel at the back. Tracking wise it feels pretty much the same as the old Naga but the surface calibration system with the new Naga definitely gives you a finer way of adjusting it, and I'd say it does feel like an improvement once calibrated.
5. Mousewheel feels very different, it isn't as wide/flat or as sturdy and is obviously part of the design to allow the mouse wheel left/right tilt clicks. The tilt click thing is awesome though and is definitely my favourite new feature.
6. The numpad feels completely different and is smaller, the buttons are closer together and are much taller with a slight angle to them. It is quite possible to press all the buttons at the same time with your thumb, I'm not used to it yet but this is the biggest change with the mouse as it feels completely different to the old numpad, the buttons are very easy to push in with an almost spring like feel and make an audible click similar to the sound of clicking the mouse. It's a very satisfying feeling to push the buttons in but I think you will be more likely to accidently push the button when you're not used to the mouse than with the old one, it is also much easier to accidently (or by purpose) push more than 1 button at once.
I will say that I liked the numpad on the original Naga, the button spacing was perfect for me and I had become very capable of using it without thinking about it, I also don't really care for mechanical keys (I don't dislike it either but It doesn't affect me either way) so I'm still unsure if the massive difference is going to be to my liking, I have to re-learn how to use it now.
7. I miss the 4/5 buttons on the side of the left click that the original Naga had, I hate the 2 buttons behind the mouse wheel and can't say I will use them, thankfully the mouse wheel tilt clicks can be assigned to that job and so I just use that instead.
It seems to me they have reduced costs on some parts to improve others (new numpad, tilt wheel and 8200dpi laser) because though I actually like the way the mouse feels it is definitely a cheaper case material to the original Naga. Razer Synapse system is very easy to use and allows a lot of flexibility, I don't think there ever was an issue here that could give an advantage to Logitech.