Calculator need more functionality particularly for conversions (weight, currency, distance, graphing etc..) Possibly even integrate with commercial calculators (TI-xx, Casio).

Windows 7 Calculator is going to include several new functions
and the Mac OS X Calc already includes these features.
http://www.youtube.com/watch?v=uqZkkqgSJ4A

I really like to see an option in calculator to switch to tape mode, like in this picture made with gimp:

http://img296.imageshack.us/img296/8210/calcnewnn6.png

The calculator has a limit. For example, it cannot give an approximation to 175! while it is only 174 multiplications. I remember that Windows XP's calculator could do any calculation (but warned when a long calculation time was expected).

If you check the "Show Thousands Separator" box, and then made a calculation, then you copied the result and pasted it somewhere else, it would be copied with the separator.

This should be removed when copying a number (like in Windows)

In gcalctool (the Calculator that comes with GNOME), there are no color on the buttons, all buttons are same.
On Windows calculator, they have red and blue colors on button, to make them look different, so its easier to use.
Example numerical buttons 0-9 have one color, while function buttons such as */+- have another color.

Very often I use the calculator for brief financial calculations and many of the times I need to either know a currency rate or the conversion rate, so each time I have to open my browser find them out online. Other types of conversions that are relevant here are conversions between different measurement systems.
Scientific calculations have a set of constants, which is a good start, but it is not extensible for some reason.

Calculator should provide a way to add new calculator modes/layouts easily. People often need ways to do custom calculations that are composed of basic operations, but a lot of numbers which could lead to major problems if typed wrong. It must be easy to do, support functions composed of custom operations/macros, and the ability to be easily exported/imported.

An example scenario is in sales. Typing numbers wrong can lead to lost profits.
+ - / *
Add/remove sales tax (1.10*operand)
Calculate Profit ((remove sales tax on Operand1) - (remove sales tax on operand2))
Determine RRP (addtax((operand1 + 5) * 1.33))

Or in aviation. Typing numbers wrong can lead to death.
AVGAS Litres to KG & vice versa
Pounds to kg
Miles to kilometers & vice versa
Calculate endurance from pounds of fuel.

i'd like to have a RPN mode for the calculator.
someting like GRPN by exemple.
http://lashwhip.com/grpn.html