Do you use a Wacom tablet GuitarTO? Support for external hardware is terrible in Aperture, definitely not Pro friendly. There are no shortcuts or even Command placeholders for individual tools and adjustments. For example, there's no shortcut for changing brush size. One of the most common actions can only be done with a mouse wheel or a touchpad. If you don't use either, you have to keep them around just for that one task. For a pro studio that processes thousands of photos, without dedicated hardware such as a Wacom tablet and a MIDI controller, that's a very good reason to go to the competition for Pro support.
What am I looking for in Aperture X? Besides a cleaner UI, performance improvements and feature matching LR, I want to be able to attach a MIDI controller to physically adjust HUD sliders. I want the industry standard Wacom tablet to be fully supported. And I know I'm asking a lot here but I want Apple to distinguish between their pro and consumer markets and communicate with us differently, to understand that professionals need to know that the tools they invest a lot of time into won't be abandoned.
I've been with Aperture since day 1 but that latter point, the uncertainty, has me seriously considering LightRoom. Like a lot of professionals, we won't go back unless Adobe gives us a reason to. This isn't a threat that I know Apple won't hear, it's an observation. Apple makes Pro software because they hope pros will use it and buy their hardware. If they make subpar Pro software, then they failed at their mission and pros will use something else.
Nonetheless, I'm confident that the next Aperture is coming but really, I -- like many of us here -- are only guessing based on hints. Some professionals can't afford to guess and really, what if Aperture X isn't up to par? We lose that bet because we waited patiently for so long working around Aperture 3's shortfalls only to be on the wrong team, relent and switch to LightRoom which we could have migrated to long ago.
Fingers crossed.