@sgunhouse there is not enough space above the tabs to click-and-drag the window around, at least on Ubuntu 16.04, the only space to click-and-drag is the space between the + tab button and the tab menu. I find this annoying for the same reasons as @dwelch mentioned. While I can change my habit to search for that small bit of clickable space, or use Alt+click to drag the window, every single other application on my machine has a title bar where I click the title bar to drag (or double click to maximize), so it doesn't make sense to interact with the Opera browser window in a different way. I am constantly moving windows around from different applications, so it is quite a nuisance to be unable to click and drag the Opera window - it really interrupts my work flow - almost to the point of making me want to switch browsers. Aesthetically it is also displeasing.
As mentioned previously, Firefox, Chrome, Vivaldi all have options to enable the title bar. I see no reason why Opera shouldn't as well. It isn't a complicated option to implement.