I don't need, nor do I want, add-on's. Please speak for yourself and not the rest of the community. People are using the tank as an example of why people need add-on's. I play a tank. I have no problem keeping aggro. I move the camera, look at the other mobs, watch other group member's health bars, etc. It's not that difficult. If you get so tunnel-visioned that you can't move your eyes around the screen while in combat, perhaps you need an eye-doctor, not an add-on.
As far as reactive-abilities go it's simple; they are no longer grey when they are usable. Again, eye-movement.