The eventual introduction of compacting and generational collection will:
Dramatically reduce the cost of GC for short-lived objects (dramatically, to the point of producing something like 5-10x wins on some of the stupidly tuned microbenchmarks typically used to compare JS runtimes)
Reduce typical heap fragmentation (essential for long-running processes - imagine a facebook/gmail tab left open for days)
Potentially improve computational performance (compacting your GC heap will improve memory locality in some scenarios, which means the CPU cache can be used more efficiently, which will make JS code manipulating small objects much faster)
Reduce average GC pause duration and reduce the frequency of GC pauses
In summary, generational collection is a big win for the things users tend to care about, if you look at what the advantages mean for applications. Compacting is less important, but tends to be nice too.
