Then you do not understand OmniFocus very well. OmniFocus isn't just an app, it's a tool that supports your todo system. That means a different approach to certain things such as not relying on specific things like OS X/iOS.
The integration with e-mail is something they simply couldn't do before Yosemite and iOS 8 since both OS X and iOS didn't support it (no share sheets and such). They can now and they are already working on it, we should see it in future updates. Contacts is the same story but Apple has already fixed it so I'm not sure why OmniFocus doesn't use it. It makes calling someone back much easier (with just 1 click you can get to the phone number). Luckily the Calendar integration is there since OmniFocus 2. It is a very welcome addition to Forecast because it gives you some more overview of your day/week.
As for Reminders...it's a different application and seems to be aimed at people who require the most simplest version of a todo application. It's just a list but one with alarms and geolocation (most people seem to want to have an app that reminds them to do something at a certain time and/or certain place; OmniFocus can't do this, it relies on you doing reviews of your todo items, it will only remind you of overdue items). It would have been nice if there were more tighter integration with Calendar and Contacts, at least when it comes to dataconnectors (Mail does integrate that way with Calendar).