Simplest explanation is almost often true - Coulson's inclusion means the show will take place prior to Avengers. When you think about how much SHIELD had going on with their Phase 2 weapons program and tracking all the different Avenger recruits, there's plenty of story to tell in between Thor and Avengers. Clark Gregg is also too much a fan favorite (and a TV veteran to boot) that they'd try to force a show without a recognizable face.
Having a SHIELD show after Avengers and not showing Steve Rogers, BW, and Hawkeye at all is too much of a stretch. Not showing Nick Fury already makes this a tough pill to swallow, unless SLJ wants to amend his contract; he takes on any role these days, maybe he'll consider cameos on TV? If the show picks up enough steam and supporting roles in the show start to gain traction, I'm quite sure they will expand the timeline to spill into the MCU Phase 2.
One can only hope. Even with Whedon involved, this smells to me like it could vary from watchable to complete trash. The MCU films already establish the upper echelon, it's going to be near-impossible for a TV show to impress us at that level unless ABC-Disney decides to throw the bank at this.