What are a few main differences between ATC interactions for rotorcraft vs fixed-wing?
A helicopter can be cleared to depart from any position on an airfield, however there are a lot of considerations that would go into that. 

There is no regulation in the US, and I'm almost certain there isn't an ICAO SARP that states a rotary winged aircraft must depart from a designated position on the airfield. Based on local conditions, volume &amp; types of air traffic, and how familiar controllers are with handling a helicopter, there may be some localized procedures, but nothing regulatory.

As far as hovering, there are no prohibitions aside from local airfield procedures. If given a hold during an IFR flight, the rotary wing pilot will fly it in the same fashion as a fixed wing aircraft. To hover in a specific spot for an extended period of time at altitude of not practical due to lack of visual cues needed to hover. Some have a system of Auto-Hover, but that's generally used for extended hovering close to the ground. 

Even if it were desired, in many cases it wouldn't be possible. Hovering requires a significantly higher amount of power when done so outside-of-ground-effect (OGE), which is a key metric you need to understand prior to every flight based on that day's conditions and how heavy you are. The extra power required burdens more gas, and in many cases requires a good percentage of the pilots available attention. 

If burning the extra gas isn't enough, most civilian rotary-wing aircraft are not equipped with the power needed. Even those military helicopters with two engines (think UH-60 Blackhawk) are limited at higher attitudes.

I think that was all the questions.