Number of Decimal Points
The default way Excel displays numbers after the decimal point needs an update. Right now, here are the results of Excel default formatting settings. (the values in brackets are in different cells)
A1 = [1.234]
A2 = [1.234]
A3 = [A1*A2] displays [1.522756]
2X increase in values displayed post decimal!
4 sig figs become 7 sig figs
A1 = [2]
A2 = [Log(A1)] displays 0.30103
5X increase in values displayed post decimal!
1 sig figs become 7 sig figs
Excel is clearly just displaying all the digits it calculates. This is not the proper way to account for significant figures, and, more importantly, it is a huge pain when working in excel. I have to click the decrease decimal button more or less every time I fill in a new formula because Excel's default only adds digits.
I nice default would just be to use the rules of significant digits which can be found online. A user can always increase or decrease around that default but in general the number of digits would not always be increasing.
Another option would be to just carry over the same number of digits.
Anyway, there are many options. The way Excel is setup right now is the worst.
3 comments

Phillip Johnson commented
I agree that consideration of significant figures (significant digits) has been missing in Excel for way too long. Normally calculations should be carried out using all available significant figures (i.e. all 15 digits available in Excel), and then only the final result for display purposes is rounded to an appropriate number of significant digits. Note that it is important that the display be rounded, while the cell value not be rounded since it may be used in subsequent calculations. I would be satisfied if the number of significant figures to display was determined by the user; but amazed if more functionality was added to trace all precedence inputs to a cell formula result and automatically round to the appropriate number of sigfigs (assuming that all cells with a number value rather than a formula have a format set to identify the number of significant digits).
Please add (1) capability to format the display of cell values to a user specified number of significant figures (also known as significant digits), and (2) functions that round, rounddown, roundup to a number of significant figures as an input argument. For reference reading see the following web page which includes links to published papers and more detailed explanation: http://www.angelfire.com/oh/cmulliss/. I am tired of implementing hacks like this: https://www.vertex42.com/ExcelTips/significantfigures.html.

Pady commented
Perfect sugestion what should be in Excel from day one.
Give us figures formating using the SIGNIFICANT FIGURES RULE. 
Peter Gorham commented
In addition, when entering or editing a formula in a cell that is already formatted (say comma2, for example), the formatting is ignored and the number of decimals expands as discussed above.
Please respect the existing formatting of cells when entering or editing.