Speed of development is one of the main advantages of using Unity. One of the most important contributors to that is the extensive list of plugins available on its Asset Store. Don't forget to check their user reviews and what version of Unity they are tested on before jumping in. Here's a few that we are already using for Defect.
We use FMOD Studio for our dynamic music system and sound effects. It gives our composer/sound designer lots of control and is fairly easy to integrate. We’ve blogged about it here: The Music System 15 years in the making.
2D Toolkit and Multiplatform Toolkit
We use 2D Toolkit's sprite system and Multiplatform Toolkit's platform specific positioning and scaling for our UI. Both of these plugins aren't platform specific, and come with code, so they're easily extensible.
4:3 vs 16:10 Shipyard
Fast Switch Platform
Switching between platforms in Unity means you have to wait for Unity to re-export all your assets in the appropriate format for the new platform. Fast Switch Platform caches this exported data so that only changed assets need to be exported. This a massive speed boost and a lot simpler and cheaper than some sort of server based solution.