Answer:
   
   The biggest difference is implicit. Both conversions require
   acceptance of the "yoke of the mitzvot"; that is, an agreement to live
   as Jews in accordance with Torah (whether or not the specific phrase
   is used). However, the interpretation of that phrase differs
   substantially from Orthodoxy (where it implies acceptance of the
   authority of Rabbinic law as well as all 613 commandments as written)
   to Reform (where it is autonomy and choice based on study). The book
   Conversion According to Reform Halakhah, published in 1990, says "[The
   phrase] 'According to halakhah' means according to our Reform Jewish
   tradition. Over the last two centuries we have developed a
   considerable body of halakhah of our own. Some of it in the form of
   books of guidance (S. B. Freehof Reform Jewish Practice; P. Knobel
   Gates of Mitzvah among others); through statements made at synods and
   conferences (W. G. Plaut The Rise of Reform Judaism; M. Meyer Response
   to Modernity), and through more than a thousand responsa written by
   Solomon B. Freehof and [others]. There is therefore a Reform tradition
   which has been expressed in an expanding halakhah."
   
   Other than that, Reform has different requirements for witnesses.
   Reform in the United States does not require ritual immersion, and
   does not mandate b'rit mila for males (although it is strongly
   recommended); Reform outside of the United States requires both
   milah/hatafah and tevilah, and tends to be more traditional in
   general.