Overall:

build: Go 1.4 required to build (https://golang.org/cl/2470, https://golang.org/cl/2993)

New Ports:
Darwin/ARM, a.k.a iOS. (https://golang.org/cl/2118, 2119, 3273, 2121, 2122, ..., 2127)

API additions and behavior changes:

bufio: add Reader.Discard (https://golang.org/cl/2260)
crypto/cipher: clarify what will happen if len(src) != len(dst) for the Stream interface. (https://golang.org/cl/1754)
crypto/elliptic: add Name field to CurveParams struct (https://golang.org/cl/2133)
crypto/tls: change default minimum version to TLS 1.0. (https://golang.org/cl/1791)
crypto/x509: wildcards are now only accepted as the first label (https://golang.org/cl/5691)
encoding/base64: add unpadded encodings (https://golang.org/cl/1511)
go/ast: add Implicit field to ast.EmptyStmt; changed meaning of ast.EmptyStmt.Semicolon position (https://golang.org/cl/5720)
log: add SetOutput functions (https://golang.org/cl/2686, https://golang.org/cl/3023)
net/http: support for setting trailers from a server Handler (https://golang.org/cl/2157)
net/http/cgi: fix REMOTE_ADDR, REMOTE_HOST, add REMOTE_PORT (https://golang.org/cl/4933)
net/smtp: add TLSConnectionState accessor (https://golang.org/cl/2151)
os/signal: add Ignore and Reset (https://golang.org/cl/3580)
runtime, syscall: use SYSCALL instruction on FreeBSD (Go 1.5 now requires FreeBSD 8-STABLE+) (https://golang.org/cl/3020)
strings: add Compare(x, y string) int, for symmetry with bytes.Compare (https://golang.org/cl/2828)
testing/quick: support generation of arrays (https://golang.org/cl/3865)

Tools:

cmd/go: std wildcard now excludes commands in main repo (https://golang.org/cl/5550)
cmd/go: .swig/.swigcxx files now require SWIG 3.0.6 or later
cmd/vet: better validation of struct tags (https://golang.org/cl/2685)
cmd/ld: no longer record build timestamp in Windows PE file header (https://golang.org/cl/3740)

Performance:

cmd/gc: optimize memclr of slices and arrays (https://golang.org/cl/2520)
sort: number of Sort performance optimizations (https://golang.org/cl/2100, https://golang.org/cl/2614, ...)
strconv: optimize decimal to string conversion (https://golang.org/cl/2105)
math/big: faster assembly kernels for amd64 and 386 (https://golang.org/cl/2503, https://golang.org/cl/2560)
math/big: faster "pure Go" kernels for platforms w/o assembly kernels (https://golang.org/cl/2480)

Assembler:

ARM assembly syntax has had some features removed.

	- mentioning SP or PC as a hardware register
		These are always pseudo-registers except that in some contexts
		they're not, and it's confusing because the context should not affect
		which register you mean. Change the references to the hardware
		registers to be explicit: R13 for SP, R15 for PC.
	- constant creation using assignment
		The files say a=b when they could instead say #define a b.
		There is no reason to have both mechanisms.
	- R(0) to refer to R0.
		Some macros use this to a great extent. Again, it's easy just to
		use a #define to rename a register.
	
Also expression evaluation now uses uint64s instead of signed integers and the
precedence of operators is now Go-like rather than C-like.