Phony Benoni: This is a great game on many counts. First, it is a struggle between two opposing plans pursued logically and ruthlessly, resulting in a thrilling conflict.
The game may have changed the course of Pillsbury's chess career. Before Hastings, he was a relative unknown in Europe. Sure, he'd had some mild successes in the States, but what great player had the U.S. produced outside of Morphy? Most of its representatives, like MacKenzie or Paulsen, were immigrants from Europe.
In the first round, Pillsbury had lost a difficult game to Chigorin. Had he lost this one as well, would he have gone on to success at Hastings? Or would his confidence have been sapped?
Instead, he went toe-to-toe with one of the top five players in the world, and beat him unflinchingly. That had to open a few eyes, especially when he kept doing it round after round.
So what about the opening, specifically <4.Bg5>? As pointed out, it had been played before, as you can see from this Opening Explorer list:
Sixty-one appearances in our incomplete database, including three times each in the world championship matches of 1886 and 1894. Steinitz seemed to be getting fond of it in the 1890s. However, the demerits of the move were well known. Here's what Isidor Gunsberg had to say about it in the tournament book:
<"No good results from this early sortie of the bishop. The attack, or, perhaps better speaking, would-be attack, differs from similar play in the French defence, inasmuch as White has not P to K5 at his command. Generally speaking, both the first and second player in this opening require their Queen's Bishop on the Queen's side.">
(One of the highlights of my chess career was finding a copy of the tournament book in the University of Michigan Graduate Library with a pencilled comment next to the note: <"Totally wrong! This game established 4.B-Kt5 as White's best move! Gunsberg is an idiot!!"> I imagine that writer is posting on the Internet at this very moment. Such creative anachronism never dies.)
Less colorful was C.E. Ranken's comment in the 1895 <British Chess Magazine>, p. 384:
<"The early sortie of the the QB to B4 or Kt5 has now become quite fashionable in this opening, but not infrequently it leaves the Q's side rather weak.">
This is, I feel, closer to the truth, and note that it's exactly what goes on in the game. The Opening Explorer offers some insight as to what came next. Pillsbury played <4.Bg5> several more times at Hastings, and Lasker, Janowski, and Tarrasch (!) also gave it a whirl. By 1896, it's in use by a wide range of players in several different tournaments. It had become fashionable.
We all know the legends of Greasy Grey-Haired Grandmasters who have burned the Midnight Oil in the never-ending quest for the Holy Grail, a <TN> next to one of their moves. More often than not, I have the feeling that an innovation is nothing more than a player looking at a position and saying, "Oh, what the heck! This looks interesting."
While submitting games from the US Opens in the 1950s, I checked each one against the Opening Explorer to avoid duplicates. I was constantly amazed how relatively unknown players would find a move that wouldn't be introduced into grandmaster play for another 40 or 50 years. I shouldn't have been. In all probability, they didn't know the book move and just played something that looked logical and didn't dump material.