SSI-2008, world average score
The world average score for the SSI-2008 is 5.7 (or to be more precise 5.65). One may be inclined to say, well that's nearly sufficient. However, bearing in mind that full sustainability requires a 10 for each indicator and thus an overall score of 10, a score of 5.7 is way below sustainability, only just over half-way.
Scores categories

The scores appear to be distributed very unevenly over the 5 categories, as can be seen in the graph above.
Indicators
The graph shows the average score of the 151 countries for each of the 22 indicators. The differences between the scores are large. The highest score is 9.2 for International Cooperation. The lowest scores are for Waste Recycling (1.7) and Consumption of Renewable Energy (3.2).
The differences underline the necessity to always have a close look at the underlying figures.
Regions
Taking a look at the scores for the 5 categories in each of the 7 regions, large differences between the regions become visible. The overall picture shows a relatively high score for Personal Development, apart from the score of Africa. On the other hand, lower scores are found for Healthy Environment and Well-balanced Society, and poor scores for Sustainable Use of Resources. Sustainable World scores slightly higher than the latter. The word 'relatively' should be emphasised: all scores are way below full sustainability. Even the relatively high scores of North America and Europe for Personal Development still show a gap to full sustainability. This means that a significant number of people is lagging behind in various aspects of Personal Development.
Income

It appears that low income countries have the lowest scores for the indicators with emphasis on Quality of Life and the highest for (most of) the indicators with emphasis on Sustainability. Exceptions for the latter are Waste Recycling and Forest Area, both not surprisingly. Low income countries score significantly better on the indicators Emission of Greenhouse Gases and Ecological Footprint. For high income countries it is the other way round. Middle income countries have also in this respect a middle position.