I like the principles behind felesan.9587’s list, and I would strongly support that kind of implementation.
As for specific instances, the only personal negative experience I’ve had has been trying to complete Devouring the Brand in Iron Marches with a friend of mine, which resulted in multiple days trying and several bug reports due to the frequent issues with that meta event that can cause it to hang at various stages. If I remember correctly we actually had to finally get a taxi to another map instance to get it finished (and then we had to recruit a few other people for assistance). It was pretty embarrassing for me since I had recruited him into the game on the promise of accessibility and reliability.
As to the broader issues, there really has to be a happy medium between the current top-heavy system, which is meant to appeal to new players, and the old system, which by design’s own admission was far more appealing to veteran players. I appreciate that you can’t make content fair for one group of players that has access to full trait trees and another that doesn’t, but remember that in addition to converting new players you also have a major source of recurring revenue in your vets, and while you certainly want to reward someone for his box purchase with a smooth first leveling experience, you don’t want to make him feel punished for buying that extra character slot once he’s on his third or fourth alt.
My view is that even in the current system, there are no new systems being introduced past level 30, when traits are introduced. For even brand new players the fundamentals of traits are going to be mastered within five or ten levels at most. The tiers themselves don’t actually require any new or different understanding with respect to traits, especially since by the time the player has hit 60 (as it is now), he or she has had a long time to play with different builds and experience the nuances of his or her trait lines, made longer still by the fact that he or she only has 6 points to work with.
Sure, the new player is still unlocking skills at 60, but as with traits there’s not a new system being introduced with those later skills—only different variations within the current systems. Further, right or wrong, the skills a player is unlocking at the upper levels are frequently being unlocked for completion or variety rather than for experimentation, as the player has already settled on the style of play that he or she prefers. That may suggest that many skills need work, but there’s no faster solution for that problem than there is for traits.
I’d like to see all traits moved forward, so that the second tier opens up at 40 or 45 and the final tier opens at 50 or 60, with corresponding trait point awards to make the tiers accessible. Then at the genuinely higher levels players can spend their time working with builds that are full or near full (and honestly I think it would be better for players both new and old to have their full allocation of trait points by 70).
For that matter, leaving a window of ten to twenty levels of no new content could pave the way for the introduction of new horizontal progression systems or for reworks of current systems that are level-gated (such as systems currently found only in items, where higher rarity is only available at higher levels). At upper levels you could go a long way just focusing players on understanding what different stat combinations mean and what varying runes and sigils contribute, and the way you avoid the mechanics overload problem is by blocking off the competing mechanics into different level blocks (skills ~1-35, traits ~30-70, equipment ~68-80).
And that’s not even addressing the possibility of totally new mechanics we haven’t seen before, such as skill customization (as per World of Warcraft’s glyphs or Diablo III’s skill runes, for instance), which could have a major impact on player enthusiasm and build diversity without an overwhelming development investment and which would naturally belong at higher levels of play.
Whatever you do, though, remember that new players that stick around are only new the first time around. They’re returning players from then on, and if a returning player is going to level 8 characters to 80—as many of us have—he or she doesn’t want to wait until the end of the journey to get the things that could best be used to make the journey more enjoyable.