IPAs have loads of hops and bitterness, whereas mild ales have hardly any.  

Here is a list of traditional beer styles and their bitterness ranges:

Beer                          International Bitterness Units
American Lagerⁱ             8-15
Mild Aleⁱ                         10-25
Belgian Gueuze            15-25
Scottish 70/-                 17-28
English Bitter                25-40
Dry Stout                      30-45
Bohemian Pilsner          35-45
English IPA                   40-70
Barleywine                   50-120
American IPA                60-120

This list is summarised from the excellent BJCP (Beer Judge Certification Program) which trains people to be beer judges in the United States.   The full text is here: http://www.bjcp.org/2008_Guidelines.pdf and is an excellent read.

You'll notice that the American craft brewers IPAs really make up for the macrobrewers generic lagers by blowing your tastebuds away with bitterness and flavour!