If im not mistaken the d90 has 4 buttons and 2 dials as opposed to just 2 buttons and 1 dial for the d5k, these 2 buttons and 1 dial can also do what the 4 buttons and 2 dials can do so that's what made it simpler and less intimidating (for me)..and about the ISO, after you configure the Fn button in your custom setting, you can adjust the ISO sensitivity the same way:
1) press and hold Fn button
2) turn the command dial till the ISO value I want
3) release the Fn button
aside from the aperture, shutter speed, exposure, ISO all the rest of the settings would be set using the menu shortcut button..as a noob casual shooter, those 4 settings are my most frequently changed and thus the dedicated control for them..
More buttons and dials is easier to use because in that case, each button and dial will be dedicated to its own function and there will be no need to browse through the menu to change the usual adjustment. In fact don't even have to look at main lcd. Like for instance if you have many buttons and dials, then maybe it will be like...
aperture: dial A
shutter speed: dial B
iso: button A+dial A
wb: button A+dial B
shooting rate: button B+dial A
focus point: button B+ dial B
flash exposure: button C+dial A
metering mode: button C+dial B
exposure: button D+dial A
You may not use all the functions today, you may not use all the functions tomorrow, but someday when you need it, it is already there and you don't have to reassign. Initially you might feel weird with more buttons and dials, especially if you don't know what each icon means, but once you get used to where they are, they make adjustment a lot faster.
I don't own a nikon, but from my little bit experience of using a friend's d90, the top lcd, viewfinder and good grip feel already more than worth the extra $300.
Btw, not sure if someone already pointed out, the STREET PRICE(I.E. THE PRICE YOU WILL AND CAN GET AT
MOST GOOD CAMERA SHOPS) of the d90 body is only around $1350. Kit with the 18-105mm is only
$16XX which is just
$300 more than the d5000 body.
Some shops can bundle the d90 body with the same 18-55 kit lens that comes with the d5000 kit. Price is usually between $1480-$1500. Just $300 or so more than the street price of the d5000kit.
If you want to compare, compare apple to apple.