Part of what makes The Flash so engaging is its connection to Arrow. I think the pilot benefits greatly from the fact that we’ve already met not only Barry Allen, but some of his cohorts as well. Caitlin Snow, the future villainess Killer Frost (Danielle Panabaker) and Cisco Ramon, a.k.a. Vibe (Carlos Valdes) all appeared on Arrow last season, so we’ve already been introduced to them and know a bit about who they are. This makes their introductions easier to swallow and allows the series to hit the ground running.
Also, the best moment in the episode comes when Barry runs to the neighboring town of Starling to get some advice from Oliver Queen. They each witness the other using their unique ability (Barry zooming off back to Central City, Oliver zip-lining off of a rooftop) and both are impressed and inspired by the other. It’s a great, perfect moment that alone justifies the fact that both series exist in the same universe.
While Smallville took ten years to show us the journey Clark Kent takes to becoming Superman, Barry Allen is suited up as the Flash and patrolling the streets of Central City in less than 45 minutes. Right away the series is telling us that this is not your typical origin story, and that The Flash will most definitely be at the forefront of his own series, costume and all.