One of the things that frequently annoys me with Excel is the position of the
tabs at the bottom of the window.
Usually columns are labeled at the top of a sheet and if I have a number of
sheets with the same or similar types columns but in different contexts I
always need to take my glance off the top and look at the bottom to check
whether I really look at the information I think I'm viewing.
I searched but I haven't been able to find any indication that the tab list
could be moved to the top to facilitate seeing the overall context with a
single look.
Most other software using tabs has the tab list ot the top or even allows
the user to decide where to put it.
Therefore my suggestion: separate the the tab list from the horizontal sheet
scrollbar and handle it like a normal toolbar that the user can reposition as
it suits his/her personal preferences.