Command + T to open new tab

Actually, Sublime Text, TextMate 2 and TextWrangler all use Cmd-N instead of Cmd-T. Even Xcode uses Cmd-N to create a new file and Cmd-T to create a new workspace tab.

I'll consider adding it as an additional shortcut.

I suspect it's just the behaviour that's slightly different.  When you have a single-document window open (i.e. not a project or folder set) these "new document" commands actually open a new window to perform the action.

Making it configurable will likely be the only way to make everyone happy, though.  :/  (I like window consolidation; I keep notes in untitled documents tabbed in a window in the left third of my left-hand monitor, the active project in the remaining two thirds of the same.)

For web browsers and such the ⌘N / ⌘T shortcut separation makes sense; you're not really creating documents in that context, you're creating windows and tabs explicitly.  In an editor, however, tabs are naturally associated with specific files; creating a new tab without a file selected in it would make no sense.

Edited to add: and thank Eris you went with tabs-as-files instead of tabs-as-workspaces and have currently avoided exposing "active files" as a discrete element to the user; it adds a whole lot of confusion in certain other programmers' editors I've used in the past.