Well, one of the greatest problems are that investments are low. With low household savings (unlike the Japanese bank) and a high rate of pensioners combined with a very high household debt and a stagnant economy this does not create the positive spin one wants on a economy.
I know and this anomaly is founded on low consumer spending and high household debt.
An economy cannot be upheld on the basis of high FDI. Although it is good for growth, it is no real indicator of the economy itself. The Japanese National Bank still holds large amounts of US Dollars, i.e. an indication that they are not likely to default any time soon. This means that Japanese firms have the chances of growth (which is happening) but it is unlikely to be a lasting event unless either a) household starts spending money and/or lower their household debt burden or b) Japanese government allows for higher foreign ownership.
Furthermore, the issues of a rising average age is not to be overlooked. The stress it imposes on the labour force and state pension schemes does take its toll.
And they have few natural resources which are unable to support the nation so they rely heavily on imports.