Not sure what camera you upgraded from, but the D300 does not do as much in-camera processing and being 12MP, it is also more sensitive... and was pretty frustrated to start of with when I upgrade from D200 to D300. It is likely not your lens problem (unless it has front-back focusing [use AF Fine Tuning] or your lens is very dirty, etc.). The initial frustration I had was at not being able to capture a shot like I did with the D200, but as I discovered along the way while using the D300, that it gives you more flexibility and adjustments, which means more control in your hands... but that also means that just out of the box may not get you what you used to have almost out of the box for the D200.
The following are some things I've realized and made adjustments to on the D300...
- When shooting an indoor concert with a fast lens and no flash and using auto-ISO with different metering modes, the camera tends to over expose the shots, and some were very over exposed. The overexposure come from the inability of the D300 to decide what ISO to use, and in some scenarios, it went to ISO 1600 when ISO 200 or 400 in that lighting condition would do. Advise is to forget about auto-ISO with indoor shooting and complex lighting that changes often.
- When shooting indoors without flash, the camera tends to overexpose the shot by 1/3 to 2/3 with matrix metering. This is easily resolved by adjusting the exposure compensation. Another way is to use center-weighted average, which is more consistent in the metering.
- The default sharpening for the D300 is less than the D200. Hence, the pictures may look a little soft, so would need to bump up the sharpening by maybe +3 in your picture control. This is more for shooting in jpegs, but some people prefer to leave the sharpening to post processing.
- With a 12MP camera, my shooting flaws + lens sharpness is more evident. This is where breathing techniques, how to hold the camera, etc. becomes more evident with the pictures taken. Also, I have found the AF fine tuning useful to adjust the front or back focusing of the lenses... and even Nikon lenses needs some adjustments.
- The LCD is too bright and does not reflect the true exposure of the picture. The picture may look alright on the LCD but when downloaded onto a computer, it looked underexposed. You would need to decrease the LCD brightness by -1 or -2.
All in all... I am very happy with my D300