Have you adjusted the sharpness setting in the D90's "Shooting Menu"? The factory default setting is quite low in my opinion.

To adjust this setting press the "Menu" button and move to the green camera icon (Shooting menu). From there move up to "Set Picture Control", move right again and choose whatever setting you prefer here, I use "Normal" but choose your setting and then click right again. From here move down to "Sharpening" and bump the setting to +6 or +7.

Be sure you to tap the "OK" button to confirm or else the camera won't store your new setting.