I understand that XeTeX was written to handle UTF-8 input better than pdfTeX. However, it seems to process documents in a slightly different way. That is, documents that don't use special fonts or need UTF-8 encoding still compile differently from the two engines.
This site contains many questions along the lines of "How do I get XeTeX to do xyz ?", where xyz is something that works fine with pdfTeX. I understand that XeTeX and pdfTeX do handle internals differently; my question is "Do they have to?" Is it possible to have XeTeX handle fonts/input differently, but once this is accomplished, it handles everything else exactly the same way as pdfTeX? Or is the issues of input encoding and fonts so fundamental that things necessarily have to be different, and workarounds must constantly be made to match outputs from the different engines?