1.4 focused on getting the GC ready for the changes in 1.5.
First, the code was rewritten from C to Go. Second, the GC was made precise, which is a major improvement to a GC.
The GC was not made truly concurrent however, and short pause times were not addressed either. These concerns are addressed in the 1.5 release.
The trade-off in 1.5 is to eliminate pause times for slightly worse throughput. For the programs Go is written to handle, this trade-off is probably fine.
