Well there are a few difference.
1) Most noteable: different fonts.
I don't know if that is correct. Unless you specified a font by name, QT and gtk may or may not have different default fonts? (probably some config for each of them on your system)
2) the "filter" text is not grayed.
I don't know if qt supports this, but if qt can do it, it seems like a bug.
3) The columns of the grid.
Under gtk the columns are extended to fill 100% of the space.
Under QT, they only take as much space as each of them really needs (IIRC that is the same under Windows).
I am not sure if GTK can be made to behave like the others (or what carbon/cocoa do).
4) the indent of text in the treeview
No idea why qt does that. Assuming none of the text label has leading spaces.