I do not come here to defrumify Isaac Leeser, but to offer a little bit of perspective on one reason that the nuances of history are challenging to Orthodoxy.
Isaac Leeser needs no introduction to one who knows even a little bit about American Jewish history.
He is the sort of person about whom the Jewish Observer prints an article Lynn M. Berkowitz, "A Preacher of the Word of God: The Rev. Isaac Leeser, " Jewish Observer 26:1 (February 1993), pp. 27-37 (reprinted in Torah Lives: A Treasury of Biographical Sketches, part of Artscroll's Judaiscope series). In fact, that's Isaac Leeser, bottom row, second square from the left:
In any event, Leeser was known as a tenacious fighter for traditional, Torah True, non-Reform Judaism in America, the sort that was called 'orthodox' or 'conservative' (lower cases both) in those days.
So what's the problem? The problem is simply that which was hinted at in the paragraph above: it is essentially that although yirat shamayim is timeless, the sort of traditional, Torah True Jew that Isaac Leeser was doesn't really 'work' in a contemporary context, and only in a 19th century American one.
One of his greatest accomplishments was his creation of the very first Jewish Bible translation in English (see). Begun in 1838, the Pentateuch portion was completed and published in 1845; Hebrew and English with haphtarot and annotations.The entire work was completed and published in 1853, in five handsome volumes, and it quickly became THE Tanakh used by English speaking Jews, a position it enjoyed until the 1917 JPS edition (which was, in part, inspired by it--it seems Bible translations have a shelf life of about 50 years, unless it's a Targum).
Since Leeser was a one man dynamo , and he produced a quality work, it's no shame to point out that he wasn't the greatest scholar (or so I am told often)--he was a talented man and a very good student. It is for that reason that he didn't invent the wheel, relying heavily on the King James Version, making changes in conformity with Jewish interpretations and tradition , as well as the modern German versions by Mendelssohn and Zunz, as well as Ludwig Philippson.
Of the latter, Isaac M. Wise wrote in his obituary for Leeser that he had to convince him to use Philippson's translation. Leeser had initially been unwilling because Philippson was a "reformer."  So that much is Torah True. But Zunz? Mendelssohn? Wos ist? How does a Torah Personality rely on them, or make a distinction between them and Philippson?
The answer is simply that this is 19th century American Orthodoxy, not 21st century.
This is not to say that Leeser therefore agreed with everything that Mendelssohn and Zunz ever wrote; obviously quoting someone does not mean that. However, it is clear that he did not view Mendelssohn or Zunz in any way comparable to 'reformers.'
He wrote, in 1829, of Mendelssohn:
"Moses Mendelssohn has done more than any other individual who has lived since the days of Maimonides and Yarchi, for the improvement of his fellow believers." 
Of course Mendelssohn's translation is cited constantly in a Torah True commentary , the majestic הכתב והקבלה, sometimes to agree and sometimes to disagree. This is another illustration of this challenge (on this, see a response here--which, by the way, really might have acknowledged that I was the catalyst for it. After all, I was referred to as "[Those] who milk the episode for its controversy," because of this and this).
There is much more to say on this topic, particularly about Zunz. I will say it eventually.
Read from the Introduction to the 1953 edition here.
See here for the interesting catalog of the Leeser Library.
 He only received slight assistance from Joseph Jaquett; besides that it was the work of one man.
 For example, the KJV translates Lev 23:15 as ye shall count unto you from the morrow after the sabbath; Leeser gives the halakhic from the morrow after the holy day.
 The Israelite, XIV [No.32; February 14, 1868, cited by Matitiahu Tsevat, "A Retrospective View of Isaac Leeser's Biblical Work," in Essays in American Jewish History, pp. 295-313. The full quote is given by Lance J. Sussman in "Another Look at Isaac Leeser and the First Jewish Translation of the Bible in the United States," Modern Judaism, Vol. 5, No. 2, Gershom Scholem Memorial Issue. (May, 1985), pp. 159-190:
"We saw him in his house. He informed us of his enterprise and of the German translations which he consulted. . . . Why do you not use Philipson's? we asked; because he is a reformer, was his reply. We convinced him, however, to the contrary in regard to that Bible and he bought a copy. With admirable skill, he used Philipson without betraying one word that this was his main authority, in the notes especially."
Actually, Leeser cites Philippson in the preface as well as the notes.
 In his The Jews and the Mosaic Law. He certainly was not oblivious to the Reformist appropriation of Mendelssohn, writing in 1839 "Our philosopher is often invoked in defence of reform, so-called, and, at times, of absolute infidelity when, in point of fact, nothing can be farther from the truth, than that he coincided with the wild schemes of our moderns, who reject rabbinical authority and tradition, not to mention that he had the fullest faith in the absolute inspiration of the Scriptures." (Quoted by Sussman.)
It is, admittedly, unclear to me how much, if anything, Leeser knew about the controversy over the Bi'ur, Mendelssohn, Wessely, the Me-assefim, etc. from the traditionalist side, whether original 18th century traditionalists like R. Yechezkel Landua (who condemned both the Bi'ur and Wessely's Words of Peace and Truth) or his 19th century contemporaries.