A. What is this?
For those not in the know, Large Address Aware is an executable flag that tells Windows that a program can handle more than 2GBs of memory. Why this flag exists and is needed is something that would take a lot of time and effort to explain, so instead I will direct you to this link (but you don't need to know this stuff to use it).
B. Why would I want to use it?
This is actually very simple. If you don't enable this flag, the game can never address more than 2GBs of memory. TOTAL memory, as in video + system memory (which are two separate things, if you don't know). As you might imagine, you can hit this ceiling in very short order, which is good enough reason on its own to enable it. If you hit the ceiling, you will start seeing "ERROR" all over textures, some outright missing textures, and general instability.
Enabling the flag prevents all of these issues from occurring. Note, however, that this only works if your PC has at least 3GBs of RAM (preferably more, because there's plenty of stuff in the background that will hog RAM) and is on a 64-bit Windows (if you don't know whether or not yours is, go to Control Panel -> System and Security -> System, or just Control Panel -> System if you have icons enabled). 32-bit users can enable it and see some benefit, but must first use a command to change the user virtual address space, detailed here.
C. Isn't this against the Terms of Service?
Yes, but like the English patch, it is very unlikely that you will ever get banned for it (actually, even less likely because it's not the sort of thing that shows up in screenshots, and it doesn't actually touch any data - it's just a compiler flag). To be honest, this is something that SEGA should have enabled in the first place, as there's no good reason not to. It's 2012, after all.
D. Okay, so how do I enable it?
Step One: Download Explorer Suite and install it.
Step Two: Open CFF Explorer. On the start menu, it will be under "Explorer Suite".
Step Three: Click on File -> Open, and direct the program to your PSO2.exe, which will be in ..\Phantasy Star Online 2\pso2_bin\.
Step Four: Click on the "+" by "NT Headers" if it's not already expanded, then click on "File Header".
Step Five: Don't be intimidated by the table; click on the cell under "Meaning" and across from "Characteristics" that says "Click here".
Step Six: Ignore everything but the checkbox that says "App can handle >2gb address space". Click on it so that it is checked.
Step Seven: Click "OK". If you're paranoid, now would be a good time to backup your pso2.exe file. (This is not necessary, however, as the game will simply overwrite the file every time it updates).
Step Eight: Save, saying "yes" to the text box that pops up asking if you would like to overwrite the file.
Step Nine: ???
Step Ten: PROFIT!
Addendum: Note that you will have to repeat this process every time game goes through an update (just like the English patch, but unlike the English patch it will work every time without fail).
Let me know if this works for you.