Combine Chacham Ovadiah's mastery of every sefer of the Acharonim of any importance with the Rogatchover's style of writing in a series of "ayins" and you will have some idea of the great Geonus of R' Stern . Better yet, open up any volume of Zecher Yosef (particularly the Maamer Al Tahaluchei Aggadot at the end of Zecher Yosef v. 4) and you will see this first hand.
He did not confine his vast knowledge within the confines of the"Dalet Amos shel Halacha". He was an expert in the "outside wisdom" as well. See for example Zecher Yosef no. 53 where he quotes Gesenius in order to identify a certain type of raisins (see the interesting introductory paragraph citing all the sources that permit citing a Gentile). He was also a great fan of R' Tzvi Chajes and he quotes him very often (in particular in his Tahaluchei Aggadot).
At one point this open-mindedness caused a bit of a controversy. In one of his letters to the Sdei Chemed, not have ing any idea it would be printed, he quoted Moshe Mendelssohn. The Sdei Chemed innocently printed this not knowing the controversy surrounding Remad, which provoked a sharp letter from the Maharsham of Brezhin castigating him for citing "maskilim". R' Yosef Zechariah printed a long and interesting letter defending himself in Sdei Cheme entry - Aba Mezakeh Bra.
Unfortunately, a fire destroyed most of his writings. See the list of the writings in manuscript in the notice in the back of Zecher Yosef. The scope of his writing is vast, covering almost every imagineable area of Jewish scholarship.
R' Stern was also on the forefront of the fight against Reform . He wrote numerous articles in the newspaper Halevanon responding to the claims of the Maskilim. This article is one example of his writing. The article clearly defines R' Stern's position, vis-a-vis Haskalah, Maskilim, and Halachich Reform.
For more on R' Yosef- see the excellent series of articles by Chaim Chamiel in Sinai - (which I hope will soon be printed as a book), a biography (that I haven't found yet) by Z. Rabiner, and an article in Yavneh (What is this?). S. Fishbane's published a very good summary in HaDarom No. 60, as well as an interesting Teshuva on Chinuch and other things.
 Shai Agnon in his "Sefer, Sappur, Sippur" (hinting at the same phrase in Sefer Yetzirah) gives a reason why he chose to write in this fashion.
 Very likely it is for this reason that Yeleg chose to position him (under the anagram "Vofsi HaKarzavi") as the symbol of fanatical Orthodoxy in his billiant but distorted poem"Kutzo Shel Yud". R' Yaakov Lifshitz (Zichron Yaakov) describes Yeleg's attempts to have Halevanon banned; apparently the liberalism of the Haskalah went in one direction only. Another arch-enemy of Yeleg was R' Yitzchok Blazer whom he referred to in his writings Yitchok Nafcha. See the memoirs of R' Yaakov Mazaah (V. 2 pgs. 38 - 45), who was influenced by Yeleg in his youth, for a very moving description of his meeting with R' Itzeleh wherein he admits that Yeleg's portrayal of R' Itzeleh was a pure distortion.
Interestingly, in Zecher Yosef on Even Haezer (published by Mechon Yerushlayim) he has a Teshuva (No. 58 - first printed in Sdei Chemed) involving a case very similar to that described in "Kutzo" (involving the yud of the name Henya) in which one sees how R' Yosef Zecharya uses the full weight of his "Kocha D' Heteira" in order to ease the plight of the poor Agunah.