And lastly, my biggest gripe with 5e: they carried on what I think was the absolutely worst idea from 4e: PCs are not part if the world. They operate by a specific set of rules that are vastly different from 'monster' rules. For example, as a PC Fighter you can't by the rules face off against an Orc fighter of similar ability. Your fighter abilities are PC only.
As somebody who hasn't yet played 5e if what you say is true I have serious issues with 5e now. Though if I were to DM I'd probably find a way to put the PC's against NPC's using PC classes and really shake things up, because I fundamentally think that this difference is a poor design choice. Is there some sort of lore within the system that justifies the difference in PC's versus monsters?
Well, there's the 'don't bury the GM under a mountain of abilities, feats, and unnecessary choices when you just want a few bandits' benefit. If you WANT to stat an NPC up as a PC with some different racial stuff or something, then you can do it. For everything else, you don't have to go through that three or four times just to have a bit of variety in an adventure. >_>
It's basically the exact same thing you're going to end up doing in 3.X, unless you have an awful lot of time (PbP comes to mind) or absolutely hate yourself: you don't NEED your big bad orc army to have fifteen levels of warrior and five feats and ability score increases and masterwork equipment bonuses etc. Get the numbers roughly accurate, and that's good enough.
None of the groups I've played in have done that, and I haven't done it as a DM, maybe that's why I'm having so much issue with it. 3.5 published adventures gave you 'Bob, fighter 5, with fighter 5 stats', not just 'Bob, no justification, stats to roughly challenge a 3rd level party'. As such we only considered naturally to keep doing so for our own campaigns. I also can't say I've ever found it particularly time-consuming and cumbersome once I got some experience with the system. It doesn't take me more than 5 minutes tops to sketch a non-spellcaster NPC. I actually find it easier to do so with class levels rather than eyeballing the numbers, because it's less thinking to do.
Let's say I do 'Bob, 5th level fighter': I have to pick his feats (I'd usually just pick a feat chain and go with it as far as possible,fill rest with generally useful stuff), the rest comes automatically (HP, Saves, BAB, Skills).
Now let's say I do Bob, no justification, stats to roughly challenge a 3rd level party': I need to decide: how many feats Bob has? Which ones? How many HP? What Saves? How well he attacks? I (purely personal preference) find this more tedious.
Anyway, back to the 5e PC vs. NPC distinction:
As a DM, you can of course give orcs Class Levels, despite there being no provision in the book to do so. It still doesn't change the fact that, unless you do them for all NPCs, most of them will still feel like operating under different rules than PCs. If you look at the NPC appendix at the end of the Monster Manual for example, many of the NPCs listed there have abilities PCs can't access. What happens when a player starts wondering 'why can't I get this ability that so many NPCs of my own race seem to have?'. It's a distincition that, to my group at least, makes the game feel less like a living breathing world, and more like a (video) game.
Could someone give some examples of which rules and portions of 5e structure are more vague than 3.5?
Granted its a simpler system, and the simplicity literally confused me for about an hour after I read most of the PHB but its well worth it when you can go a session without players interrupting to ask semi-obvious questions or people disputing a rule that that doesn't exist anymore.
To give yo a quick example I enjoyed discussing over some beers with my friends: there's a significant portion of the Druid class that's so vague it makes building your character and playing it impossible without a lengthy discussion with your DM:
-There's explicitly stated what happens with your normal form gear when you use Wild Shape, but no word regarding what happens with any gear you put on in Wild Shape when you revert to your normal form
-There's no explanation anywhere how natural armor works
-There's no clear answer whether for example taking the 'Bite. Melee Weapon Attack: +5 to hit etc.' action as a tiger constitutes taking the Attack action for the purpose of rules that key off taking the Attack action.