For tpl_form exactly as you said <P> tags are changed to <table>, <tr>, <td> due to some templates incompatibility I've seen in IE6 for RTL when <P> is used. ( and so using tables makes interface more backward compatible)
For style.css actually it's not just changing font, changes are listed below:
text-align: left and right change according to RTL
background-position: left -> right
padding: change according to RTL
floats changed according to RTL
margin-left and right changed according to RTL
Fix background images shown under text instead of beside text.
Live example with new default_rtl layout can be seen here ( compatibility with all browsers tested)
Demo of New RTL layout