You get the technologies that have already been researched by every great power. If there's a great power who's far behind in research (for the Japanese it will be the Ottoman Empire if they westernize too quickly) you'll be severely hampered in your initial technology because of them.
This is not true as of HoD. You can see this thread here:
http://forum.paradoxplaza.com/forum...rmines-which-techs-you-gain-when-you-civilize for which I've replicated the results, and is one of the common myths about Civilizing. It's quite easy to replicate yourself, too: just Westernize as unsphered Japan in say, 1870, with the absolute minimum amount of reforms necessary, observe your techs and then observe those of the major powers. You will still be behind even the most laggardly GP in some cases. As far as I have worked out:
When unsphered:
You are guaranteed one technology on Civilizing. For every military reform you pass up until 5 reforms, you gain an extra 4 technologies. For every economic reform you pass up until 5 reforms, you gain an extra 4 technologies. Additional reforms in the same category after the 5th have no effect.
What I haven't worked out: what determines which technologies it is you get.
When sphered:
You are guaranteed all of the level 1 technologies (that's 25 technologies). You may get some additional level 2 technologies.
What I haven't worked out yet: what determines which level 2 technologies you do or don't get. It's some function of a) the techs of the GP who sphered you has, and b) the number of reforms you passed in each category.
Note that contrary to the usual advice, this means that Civilizing outside a sphere is normally better if you manage to Civilize in the early-game (before approx 1865). The typical path to reform is the 4 military reforms which give more Research Points on Conquest, plus the 3 economic techs which give you Railroads, a Cement Factory, and a mild literacy boost. That gives you 29 techs, compared to 25 for being in a GP's sphere. You don't start getting more than 30 techs for being in the tech leader GP's sphere until around 1865 or even later.