As someone who is returning to HERO system after decades away from it playing other games, I think that it is a hard system to learn. There are a lot of things about it I have always admired.
One of the key strengths of the system is the flexibility in character builds. So the temptation is to start straight in with that. The trouble is that character building is complex. And it is made much harder to understand if you're not already pretty fluent with the basic mechanics. Returning after a long absence, I have had a lot of trouble working out how to build powers involving senses/ enhanced senses, because the language used to describe those things has developed so much since the early editions.
So I think that even experienced gamers coming to HERO for the first time would benefit from playing a few sessions (or even just combats in isolation from a story) with pre-generated characters until they are fluent in the basic mechanics. The ones I am thinking about are characteristics, attack rolls, effect rolls, skill levels, phase chart, phases, targeting, basic combat manoeuvres, END, Stun, Normal (counting BODY) and Killing Damage, Resistant and non-resistant defences, CON-stunning, KO, recovery, Presence Attack and some of the common, easy to adjudicate powers; multi power. Less experienced gamers probably need this broken down quite a bit. If you can work in some stuff around disadvantages that affect characters in play, that's a bonus.
You also need to cover the concept of 'special effect', from the point of view of 'It's mechanic called a Blast, but it represents a Repressor Ray'. This is a touchstone of HERO games that has not become universal in RPG design. By comparison the concept of a character disadvantage is now shared by many RP systems.
Things like skill rolls, complementary skills, the time chart etc. are pretty similar to many other RPGs, so experienced players won't need much practice on them. But players new to role-play would.
- recognisable archetypes the players are going to find attractive
- simple powers, with few advantages and limitations that show off the mechanics you want to teach
- relatively short list of powers
- powers that represent easily recognisable effects from fiction
I know this might sound like a bit of a drag, but provided you have a reasonable adventure to run, the characters shouldn't need to be mechanically sophisticated for everyone to have a good time.
You might consider replacing the usual experience system with an accelerated version where each character has a list of available improvements (minor powers, extra dice, skills or characteristic points). After each adventure the player gets to pick one or two improvements to add to their character for the next session. That opens up a discussion about any new game elements they introduce, or the mechanics of how they work etc. and introduces new material more quickly than would happen if the character grew organically.
I think you're best going with simple archetypes rather than complex characters. Archetypes are good because it's easy for the player to understand what the mechanics are representing (they already have a fictional reference point), so it helps them build an understanding of how the mechanics relate to the fiction. Being familiar characters, they're easy to role-play, leaving more brain space for mechanics.
Simple characters are good because they let the player focus on learning the underlying mechanics, rather than drowning in choices. If the play group as a whole covers a wider range of powers that's all to the good. People can learn by watching others apply mechanics almost as well as they can by using them for themselves. In early games it can increase the interest of other players' turns, because the new player wants (hopefully) to understand the other player's character too. This also showcases the breadth of the system, which is one of HERO's strongest points.
You might consider inviting players to swap characters mid-session or between sessions during the learning phase, so they can try out the different mechanics, too. It would depend on the players whether this idea went down well, I guess.