Put quite simply, the Rambam's Mishneh Thorah is not only the most complete, but the most simple and easily understood of codified halakhah ever written.
Although he is often portrayed as an innovator when it comes to codification, this is simply untrue. Rather, he should be properly seen as capstone - the makeh be-faTish as it were - in this endeavor.
- Rabbenu Hananel restated the Gemara and included many quotations from other works of Hazal, most notably the Talmudh Yerushalmi
- Rav YiSHaq Al-Fasi (the Rif) - a student of Rabbenu Hananel - in his Halakhoth restated the opinions of the Gemara which he deemed to be the practical halakhah, leaving out all else but including some explanations of the text in places
- The Geonic codes, such as the Halakhoth Gedoloth of Shim^on Qayyara and the Halakhoth Pesuqoth of Yehudai Gaon, simply restate the final decisions of the Mishnah and the Gemara with some expansive comments
The practical problem of all of these codes is that they were either incomplete (i.e. only written on areas of halakhah readily applicable to Diaspora Jewry) or they were written in a mixture of talmudic Aramaic and Arabic - or both. This made them useful as references for scholars but of little use to the general population of religious Jewry who often possessed only a basic education in Humash, Mishnayoth, and their respective dialects of Hebrew.
Many of the other codes, such as the Rosh and others, were not really "codes" at all and were only commentaries on the work of the Rif.
The works of the Tur, Beth Yosef, and the later ShulHan ^Arukh, were more akin to attempts at commentary rather than codification. And even that halakhic material which is codified is often not conclusive, but cites various opinions (i.e. yesh omrim). This, in addition to similar language barriers as the others, likewise makes their works difficult to access and to practically integrate.
In more recent times, the very important and impressive work of halakhah by HaRav YeHiel Mikhel Epshtein z"l - the ^Arokh HaShulHan - was published. Although it is complete in its scope and extremely useful for a deeper and broader understanding of the halakhic discussion underlying any given subject (as well as a liquT of all relevant sources), it too is written in rabbinic Hebrew and in the Aramaic of the sources which it cites. Further, its legal decisions are based in large part on the daily realities of Lithuanian Jewry, making it in many ways inapplicable for other Diaspora communities.
What the Rambam did - and what makes him so unique as to engender a movement in connection to his halakhic code - is as follows:
- Complete: The 14 books of the Mishneh Thorah comprise the entirety of halakhah for all ages and epochs of Jewish existence, both inside and outside of Eress Yisra'el.
- Accessible: The Mishneh Thorah is written in a largely simple and straightforward dialect of Mishnaic Hebrew and the material is organized in a logical and topical fashion.
- Transcends Local and Later Customs: The Mishneh Thorah is a work that usually quotes the relevant Mishnayoth verbatim, translates the relevant passages of the Gemara and then quotes them verbatim, and also includes simple explanations and reasoning based largely on the consensus of the Geonim. Although the Rambam will at times mention the customs of Babylon, Israel, and even those of his own teachers in Spain, his work is virtually bereft of local and later customs developed in the exile. This has a unifying and simplifying effect that is unparalleled and unmatched.
- A Rational Approach: This aspect of the Mishneh Thorah has always attracted adherents, however in today's culture of information and a desire for religion that makes sense, it stands alone as an expression of authentic halakhah and ethical teachings that is thoroughly based in reason and practicality, its attraction is all the more so. It lacks mentions of evil spirits, demons, segulas, reincarnation, instructions on the efficacy of amulets, and the like. Rather, it attempts to rid common Jewish practice of such superstitious elements that had crept into Judaism even in the time of the Rambam.
- Reaches a Singular, Confident Conclusion: Far from the later trend to try and be careful to respect and fulfill all halakhic opinions [or as many as possible] simultaneously - even rejected opinions from the Mishnah and Gemara themselves - the Mishneh Thorah brings one exclusive and confident conclusion in almost every case it contains.
The reason why his work is most popular these days among converts and especially ba^alei teshuvah is because by utilizing their basic knowledge of Hebrew they were exposed to as a child, a good dictionary, and a shorter amount of time than pouring themselves into a Daf Yomi program, they can understand the halakhah. Not only this, but they can master a lot more and without the confusion of the back and forth of the Gemara.
Essentially, the vision of the Rambam is beginning to find fruition in our times. May it be to his merit.
[For a full explanation of the purpose and intention behind the authoring of the Mishneh Thorah, read the haqdamah to the Mishneh Thorah.]
Hope that this helps. Kol tuv.