I like to find and discuss underrated things.
IMHO the most underrated aspect of the works of the Choffetz Chaim Z"L is the structure found in several of his classics
1. Shemiras Halashon is an amazing anthology of Aggadah, Midrash, Zohar and Mussar on a single theme. This to me is an excellent paradigm of logically assembling sources to make a point. It reads quite smoothly.
2. The Sefer Choffetz Chaim. The opening begins with a "Sefer Hamitzvos " on the topic. He them proceeds to pasken and to provide either his sources, or his sevoros, to back himself up. Great combination of Halachah P'skukkah with Mekoros by the author himself.
3. The "Mishnah B'rurah". The idea of a bi-level Halacha Sefer may go back to the Rashba's Toras Habbayyis Ho'oruch v'hakatzar. Also D'reesha P'reeshah, et al.
The M"B's 3-tiered approach - which included adding "Sha'ar Hatziyyun" -. seems an innovation of his. B"H the Oz v'hodor edition (and perhaps others) has expanded the mar'eh m'komos to an even more user-friendly level.
There are already newer Seforim in our day that emulate that 3-Tiered approach (EG Badei Hashulchan)
Probably not since the Rambam, has an author put in so much energy into the structure of the presentation.