Overall it was quite underwhelming: the major achievement as far as climate change goes was getting an agreement to reduce emissions by 80% by 2050 and so prevent the rise in temperatures beyond 2 degrees Celsius. Before the summit, this had been Barroso's stated aim, but, though achieved, a political and non-binding agreement with no detail to have something done in 41 years time is hardly an impressive or ground-breaking achievement. The talks will have been useful in preparing the ground for Copenhagen in December; it's a weak diplomatic weapon, but the agreement could be used to urge greater action this winter. There will also be an institute to look at carbon capture: the "Global Carbon Capture and Storage Institute", which will be based in Australia. I don't know any of the details behind it: budget, remit, etc. and it's likely that all of that is still to be finalised; still, it doesn't take too much imagination what sort of stuff it will be doing.
The lack of action an real targets has led to a rubbishing of the G8's conclusions - even Ban Ki-Moon, the low-key Secretary-General of the UN had some strong words:
"“The policies that they have stated so far are not enough, not sufficient enough,” Mr Ban told reporters. Referring to the UN Intergovernmental Panel on Climate Change (IPCC), he said: “This is the science. We must work according to the science. This is politically and morally imperative . . . for the future of humanity.”"
When it comes to the financial crisis, it doesn't look as if there's much new here: just a commitment to getting credit flowing again and to try to make the Doha round of trade talks a success - since I haven't heard of any country taking the opposite line, this doesn't especially strike me as an impressive diplomatic victory. The G8 also considered the issue of food security and aid, and Obama has spoken about Africa being self-sufficient. This has resulted in a promise for $20 billion (around €10 billion) to be invested in rural aid over the next 3 years in developing countries, though, again, there will be the small matter of following up on the promise now that the leaders have left L'Aquila.
As for the significance of the G8, I'm torn. On on hand, it has become obvious that power in the world has shifted so that the G8 can no longer be expected to come up with the solutions to the problems of the day anymore. This can be seen from the simple fact that the G5 were invited along to attend. On the other hand, there is a value in a group of (mostly) like-minded countries coming together to discuss various issues at the highest level. It helps grease the wheels of diplomacy (though I suppose, depending on the leader, the foreign affairs officials may see that differently), and could help co-ordination and lead to better achievements at more specific summits. It may be the end of the G8's significance, but that doesn't mean that it's the end of the G8's usefulness.