Excel's default charts have to use the ugliest settings I could possible imagine. They break nearly every rule of effective information design by default.
- Heavy use of ink in the plot background (grey background, solid black gridlines, solid black axes)
- Colors undergo crazy variations (blue, pink, teal, yellow)
- Data points in charted lines are huge - emphasizes the data points over the lines. In most cases the trend is what you're interested in, not the individual data points.
- Similarly, data point shapes change, from diamonds to squares to triangles to crosses. Twenty years ago when everyone printed in black and white this may have mattered more, but today most data is printed in color or shown on screen. The data shape variations just adds more complexity and focuses users on the data, not on the insight between them.
Pointed Haired Dilbert has posted a nice link on how to define user-defined charts. Say no do default charts in Excel.