What is a system? Basically it's a setup that certain players use practically regardless of what their opponent plays. There are positives and negatives to such an approach. Firstly it takes a lot of the opening work out of chess, and if you use that time studying the end or middle-game, I'd say it's a better use of your time than learning every possible variation after e4-e5. Another advantage is that systems are usually very very solid, and that's why they can be used against almost anything. In the London System no weaknesses are created by white in the early parts of the game and that presents a unique challenge to black- the challenge of patience! A lot of players will eventually play aggressive, weakening moves while white just slowly develops through the middle-game.
The negative side is obvious, the rigidity of it. Most strong players agree that it's important to understand a wide variety of setups or more importantly the ideas behind them.
FM Grant Szuveges suggested the following system, which is full of imbalances. Black needs to be comfortable playing a standard fianchetto defence like the KID, or Gruenfeld in case white transposes with 3. c4, but "London players" aren't known for their love of transpositions!
If white doesn't castle and fixes the center with c3, black might have to
defend a strong attack on the weakened light squares and up the h-file.
Here's an example from two of Melbourne's own: Ascaro Pecori playing white against Domagoj Dragicevic
Pecori - Dragicevic: What is better, Qb3+ or Bc4+?
Next let's have a look at what happens when white Castles Kingside.
Finally, here's my game from tonight! Not bad for a first try, but I got mixed up recognising what opening I was actually playing after it probably transposed from a King's Indian Attack Reversed against the London System, into a King's Indian Defence proper.
The end of this game gives me a bit more faith in how I'm studying and shows the difference between studying tactics and attack. The Nxd5! idea is not really a tactic- it's not "AxB, CxA then D is hanging" - it's based on understanding that white's knight on d2 blocks his defence along the second rank. It can move, but then it gives up the defence of f3. Understanding attack makes full use of all those problems in tactics books you've solved, as they'll never appear if you don't know how to get there. Think of a Samurai who knows a thousand ways to kill a man from a yard away, but took a few weeks off Samurai school when they were going over how to get inside the sweet sweet kill-zone. That's how most chess players have studied tactics. Gotta learn both people!
I also looked at this briefly for whom it may interest! (not me!)
Preparing for the London I found it very frustrating that I couldn't seem to create an interesting, imbalanced position no matter what I played with Black. Here's what Roman Dzinishvilli recommends in "openings for black explained" with some annotations I made while trying to understand it!
What I don't like about this is that I really don't know what to do in the middle-game. It seems to me that Black has set up like a Sicilian, but with none of the logical counter-play against e4 or on the c-file that makes the Sicilian setup sensible.White has some small attacking plans against d6 and creating some weaknesses with a2-a4 and black even needs to defend economically with accurate moves in some variations.