Perhaps most significantly, the Apple ][ came with "slots". We're all
familiar with computer slots now, but it was quite innovative at the
time. Many Apple users discovered that they needed to plug extra
boards into the slots to achieve a "business-like" configuration -
things such as 80-column video, or upper/lower case (the basic Apple
came only with upper case). This sort of thing upped the price
considerably, but sometimes achieved things that couldn't readily be
done on the PET, such as 80 columns. Other things - such as upper and
lower case, or a floating point Basic - were standard on the PET but
extras on the Apple.