First a little background:
I played that other game from the early 80's to 3.5e and bought 4e, hated it then kind of dropped out of RPGs for years until picking up Pathfinder a little over a year ago. I bounced around for a few months in groups that couldn't stay together until I gave up and started my own. I have a group of player, some of which have never played any RPGs in their life before so these and despite all those years PLAYING I'd never run a game... none of us are really what you could call an EXPERT at the ins and outs of Pathfinder rules. (we even started out just using the Core Rulebook and I've kept what we added to a minimum beyond that so as not to overwhelm the new players) Additionally I've been playing an organized player character at my local hobby store for about 2 months now (just hit level 5 on my -1!)
On to the meat:
"Streamlined Proficiencies" - Does this means skills? Because I've never had a player have any major issues with skills... there really aren't that many of them (especially after some were combines from 3.x already) and their pretty self explanitory as to what they do if not how exactly the mechanics work.
I'll tell you along those lines the single biggest issue I find new players have is the HUGE number of FEATS. They look at that giant list of feats in the the Core Rulebook and throw up their hands... and feats are added in LOTS of later suppliments to the point where noobs just have no idea what to do. A fair number get annoyed later when they find out feat X would have been really good to take but they had no idea it existed in the vast multitudes of feats available. In the old days noobs used to play Fighters because they were the easy entry level class... now not so much. Fighters are Feat machines and with the insane number of feats noobs have to decide every level which one they are going to pick... it's an exercise in frustration.
I've never had a player have an issue with initiative... especially after they got rid of the reroll each round in earlier editions. The "actions" IS something that confuses them (move, standard, swift, etc. etc. etc.) as a result "Simplified Actions" sounds great!
After 4e "Easier to Play" scares me. That's where that other game went with 4e and 5e is supposed to be ok but offers less customization, I'm playing Pathfinder for a reason. I hope you don't stray too far.
We play a homebrew campaign so "Golarian-Infused" isn't useful and if it's so tightly infused to the point of making it difficult to play non Golarian games that's an issue.
I hope you break it back into three core books. The Core Rulebook is very large and can be a bit intimidating to new players if you tell them that's the basic book if they want to buy a physical copy. Telling them that if they just want to play they don't need the second half doesn't help much as then they ask why they'd buy a book they only want half of. People like to have the physical book and so a Player's Handbook is a good thing.
Keep the basic d20 based mechanic and fix where it's broken. Specifically why are there percentage checks where the increments are always divisible by 5 (if not 10)!?!? Arcane Spell Failure, Concealment, etc. would be easier to understand if they were just more d20 rolls.) If you're going to use % for things it should be more granular than 5% increments. Guess what, there are exactly 20 5%'s in 100%.
Not sure what needs fixing in multi-classing but my new to RPG players haven't had any issues with it. Contrary to what may be believed though new players don't jump at Archtypes and Backgrounds currently as the number of classes in the core rulebook alone are a lot for a new player to take in. Explaining the conceptual difference between a sorcerer and a wizard or a fighter and a barbarian, while simple to grasp for people who've been playing for a long time, is not so much for brand new players... adding archetypes for each class just makes it worse. Gone are the days of Fighter, Wizard, Thief, and Cleric. Heck some of my players couldn't figure out why they didn't just make a Barbarian a set of feats a fighter takes, a Paladin a multi-class cleric/Fighter, a Bard a Sorcerer/Rogue, etc. Adding the Hybrid classes from the Advanced Race Guide just made the situation even worse. What's the difference to a new user between a hybrid and a multi-class... it just seems unnecessarily complex.
10th level spells doesn't seem that great... it's handy to be able to roll 1d10-1 for a spell level. There are already 10 levels of spells, they just start at 0 and go to 9 intstead of 1-10.
Anyway that's my $0.02 for now. If you made it this far... WOW. As the title says I'm cautiously optimistic (I don't think an edition every 10 years is a bad idea actually but I feel burned by that other game after they made the game I loved "Easier")
