[mGGPrometheus]

Just adding on to what NighTerra said,

If protoss goes nexus first I consider it a build order loss if you go 1 rax FE and don't have an early 3rd CC. With 3CC you will have pretty much everything else exactly the same at 10 minutes, except you will have an in-base 3rd ready to float out and many more SCVs. Essentially you're 1 base ahead for free.

Many times when you lose a game and don't know how, the hidden killer is macro. If your opponent plays greedy (if protoss no-gas expands), you must play greedy or exploit greed (although aggression usually involves 13gas and that occurs before you scout). If you play safe you will be behind.