I recently migrated over from Notepad++ (and from Windows) and I am finding myself at an inconvenience without CTRL + T in Notepad++ which switches the current line with the previous line. I tried using Move selection (CMD + CTRL + ARROW), but it does not behave the same way.
It's the same as in Notepad++, control ⌃+T. Place your cursor at the beginning of the second line, press ⌃ T, and the two lines will be transposed. Your cursor will also move down a line as well, enabling you to repeatedly transpose to move a line down several spots.
You can also use control ⌃+↑ and control ⌃+↓.
Works from any point of the line, but will yield strange results if the line you're on has no line ending character.