It's the iMac's protective front glass that is causing the additional glare on the screen.
The actual TFT LCD screen is glossy, itself, and the additional front glass and the air space between the actual LCD screen and the protective glass causes extreme glossy look and glare and a slightly out of focus look.
Do as WZZZ recommends and look into purchasing a MacFrame.
I, absolutely, hated the glossy iMac screen and was getting serious headaches with only a short time of sitting in front of my iMac. As soon as I bought my iMac, I started my hunt for a good solution and found the MacFrame. Since removing the iMac's front glass and install a MacFrame, I can work at my Mac the way I used to.
The removal of the extra pane of glass doesn't completely get rid of the glare ( again, the LCD, itself, has a glossy, glass panel), but reduces the glare down to a more tolerable, and less eye straining level.
The MacFrame is just a black painted metal cover that covers over all of the ugly parts that show when you remove the iMac's front glass panel.
The MacFrame kit gives you everything you need to remove the iMac glass and re-install the MacFame in its place.
The difference in reduced glare is very noticeable and the screen actually looks clearer and sharper.
Store the original front iMac glass to protect it from getting broken or damaged.
You will need to reinstall this glass if you ever need to bring your iMac in for any servicing.
Good Luck!