I was a loyal d90 owner, having bought it when it came out. Although I tried the d300(s) cameras I never gave in to NAS as they didn't offer me anything that was going to improve my image quality over the d90 (my view was the greatest advantage the d300(s) had was the AF and burst shooting, something that was not a huge deal for me).
If shooting in low light is something you do often, than the d7000 will give you notable improvement over the d90/d300(s). The d90 and d300(s) were good up to ISO 800, 1600 if you were pushing it. The d7000 is good up to ISO 3200, 6400 if you are pushing it and in fact 12800 retains good colour and detail although it is noisy.
In addition, the d7000 would give you usable video (the d90's video mode is so bad I never used mine), a slightly better AWB, a more intuitive matrix meter (no longer will it over or under expose because it is weighing the scene towards whatever is under your focal point), and a much, much better AF system that performs noticably better in low light.
There are also little things to consider like having the option of release priority for AF-S, user shooting banks which are actually well done and useful, etc.
I could go on, there are numerous things about the d7000 that make it an improvement to the d90 and the d300(s) imo. It has the capability of certainly providing you with greater flexibility in your shooting and certainly in low light, the ability to capture photographs you may otehrwise not be able to.
I was a d40 user who bought the d90 amidst complaints it was producing soft images. Personally I have not had a problem with the d7000 using the same basic shooting rules I used on the d90. For focal lengths up to 200mm I like to shoot at just over 1/focal length if I can, bumping up ISO if required. I have certainly captured sharp images below that with the d7000. Over 200mm I start trying to get my shutter speed up to more like 1/1.5x focal length or higher. I have never had a problem. In fact, I find with shots of highly detailed items, that the d7000's extra MP's can actually produce sharper results when treated correctly (I prefer to do all of my sharpening post processing, but I have produced great results out of the camera setting sharpening to 6 or 7).
Although I am sure some people have had legitimate issues with the d7000, such as the odd defective camera, most of the issues I have seen from people complaining of softness either involved photos taken at shutter speeds that were too low or photos that had little to no sharpening applied. Nikon, for years now, has produced DSLR's that are very conservative on sharpening to allow the user to determine how much to apply. This may cause some people to believe that a largely unsharpened image is soft.
I still think the d90 is a fantastic camera and if I couldn't afford to upgrade to the d7000 I would still be using it happily. The d7000 however definitely improved on it in areas that make a difference to me, and this was enough to pull the trigger (for me).