However, I also haven't seen issues with new people learning 5E whether there are people to help them or not. I've introduced several newbies now and they had no problems picking up the game. It's certainly a lower bar than 3.5.[/qoute] 5e is not as bad (or good) as 3.5 along a number of dimensions, sure, 3.5 was a high-water mark in a lot of ways.
I have seen new players come up hard against playing the character they want to - wanting to play a fighter type and being disapointed in how fast they died, or wanting to play a wizard and being unable to cope with the neo-Vancian magic sub-system (heck neo-Vancian has been the one sticking point for a few long-time & returning players, too). I've also seen plenty of players take to it very well with a little help. But, the issues of new players are minor compared to the duck-to-water way returning players take to 5e. It's a thing of beauty, really. WotC has gotten the balance of accessibilty to the new vs acceptablity to the old just about as right as could be hoped, and at just the right time.
You saw that aspect of 4E in a more positive light than I did. In my experience having detailed powers with no common foundation made things more difficult, both as a player and a DM. I don't know how many times we had to stop the game and pause for a few minutes while the DM read through the power trying to understand what it did.
"No common foundation?" Not sure what that means.
Powers were very plainly presented, I've seen players go nuts trying to figure out what a power 'was supposed to do,' ignoring the clear, simple text that said exactly what it did the whole time. Mostly the more serious 3.5 types.
Complexity, even if well explained, can still be detrimental to a game.
It can, but it's unavoidable in an RPG - even RPGs that keep their systems very short in terms of page count, load a lot of complexity on the GM or players. The idea that 4e is any more complex than other eds is off, too. It's actually less complex (clearer & more consistent) in structure & play. It's just much less familiar, and if D&D has been second nature to you for years or decades, you become inured to it's complexities - so a new D&D like 5e that hasn't changed them much,
benefits from that as the accustomed complexities fly under the radar, while a radically different one like 4e pushes all the complexity it does have right in your face, like a durian-cream pie.
If you consider a game who's turns can take an hour or more (I think our record was an hour and a half) at higher levels to be "smooth" all I can say is that we have a different definition for that word.
IMX, turns took as long or longer in 3.5, and more of that was dealing with rules issues or one player monopolizing the DM's time, while in 4e the time was actual play that engaged more of the table. A turn cycle could take a while (never an hour, mind you - whole, big level+ set-piece combats take an hour or two), and one 3.5-veteran powergamer who was in my campaign for a while notoriously took a couple of "15 min turns" when he busted out summons with his Wizard(Witch) Vile Scholar build, but that's about as bad as it ever got.
Really, the speed issue is more about perception, and who's taking the time. In 4e, every character can take a pretty substantial turn if they spend an Action Point or use a more interesting power. In other eds, it's casters who eat up time on resolving high-impact/complex spells (OK, and rules debate's and mapping, Joel, mapping..*). The 4e phenomenon can lead to a spiral in which players who have waited, disengaged, a little too long for their turn decide to take a really /involved/ turn to make up for it, which of course, means it's that much longer for the next guy... the solution I found to that issue was engagement. Ironically, that could mean encouraging off-turn actions and abilities that benefited other PCs, because they keep the player with them focused on the action when it's not his turn.
*[sblock] off topic but a couple of the players in our old 3.x campaign got so, so sick of mapping (which the DM & I were just happily wiling away the hours with, in loving detail, because old-school), that he essentially invented the skill challenge about 6 years early, and, eventually, created the most-beloved NPC of any campaign I've ever encountered: Gimble the Gnomish Master Cartographer (Thank you, Expert NPC class!). When Gimble was eaten by a purple worm, we went ballistic on the thing, cut his corpse out of it, and rushed to nearest high level priest to have him raised (our resident dwarf cleric made the mistake of a dipping a level of fighter, or we might not even any of us been 9th yet, but for whatever reason we couldn't do it ourselves), no debate over how far behind the wealth/level curve we were already, we ponied up rather than lose our GPS. [/sblock]