Give Feature Feedback and Suggestions
Without the plus it doesn't look like a button. It's just a blob of different colour.
For people using Aero on Windows, the Chrome taskbar might have many other blobs of colour from things showing through the aeroglass.
Also, haasboer, I don't think anyone here is saying "I don't know what the button does anymore" but rather they are suggesting that the lack of + will make it more difficult to open a new tab, especially if you don't already know how.