Apparently, this is a feature a lot of people appreciate, since both Internet Explorer and Firefox have included it in their recent releases. Personally, I hate taskbar tabs. It almost seems counter intuitive as it eliminates the advantage of organising pages into tabs and windows. In fact it pretty much nullifies the benefits of tabbed browsing. I had disabled this feature almost immediately in Firefox and did the same with Opera 10.5.
Opera 10.5 doesn’t offer any option to disable taskbar tabs, however you can get around this limitation with a simple trick.
Browse to the location where Opera 10.5 is installed (default: C:\Program Files\Opera).
Right Click on opera.exe and click on Properties.
Browse to the Compatibility tab and set it to run in capability mode for Windows Vista.
Apply changes and launch Opera. Taskbar tabs should now be disabled.
Update: This trick is no longer neccesary, since Opera now includes the option to disable taskbar tabs. Click here and uncheck the “Use Windows 7 Taskbar Thumbnails” option. (Thanks Paolo)