Jason, the most noticeable difference between quirks and standard compliant mode in IE is, that IE has treats the box model wrong, when in quirks mode.
According to CSS 2 standard, margin and border do not account to the width of an object, so the overall width is margin-left + border-left + width + border-right + margin-right. When in quirks mode, IE substracts margin and border from the object's width, which might have quite a massive influence on your design, even (or especially) if your CSS is absolutely fine.
Furthermore, in standard compliant mode IE no longer knows some of it's proprietary extensions, like the use of JavaScript expressions in CSS (which won't bother you, of course, if you stick with standard compliant CSS).
Jason, the most noticeable difference between quirks and standard compliant mode in IE is, that IE has treats the box model wrong, when in quirks mode.
According to CSS 2 standard, margin and border do not account to the width of an object, so the overall width is margin-left + border-left + width + border-right + margin-right. When in quirks mode, IE substracts margin and border from the object's width, which might have quite a massive influence on your design, even (or especially) if your CSS is absolutely fine.
Furthermore, in standard compliant mode IE no longer knows some of it's proprietary extensions, like the use of JavaScript expressions in CSS (which won't bother you, of course, if you stick with standard compliant CSS).