To [MENTION=12630]Ahrimon[/MENTION] and [MENTION=92511]steeldragons[/MENTION], you make some good points. A lot of rules are explicitly called out as optional and say "check with the DM" in the 5e PH. But I do feel like the weight of history behind D&D is bearing down on the PH here. In my experience (your mileage may vary), the PH has always been considered almost entirely non-optional. Ability score generation is one of those things that DMs have, historically, house-ruled because it can set the tone and style for an entire campaign. But aside from that one example, I think any D&D player, regardless of which edition he/she is most familiar with, sits down at the table thinking that the PH rules are going to be taken at face-value unless the DM says otherwise. I've been a DM to dozens of players in various cities and states over the last 15 years (hundreds if you count organized play but those games have very clear rules for chargen) and the only thing players almost always ask when playing in any edition is "how do you do ability scores?" The second-most common question I get is about starting equipment, and even then they usually know going in what the answer is likely to be. They have never asked specifically about spells, combat, classes, or anything else. These things are generally lumped up together in the question "Anything else?" under to the assumption that all other rules are sacred unless I say otherwise, and that is if they ask about anything else at all. Again, YMMV, but this is the consistent experience I have had (and not just in D&D but any RPG with a core rulebook).
The 5e PH is worded much more strongly regarding what is "optional" and what it not, and WotC has emphasized the modular nature of 5e since the beginning. Nevertheless, it is my firmly-held belief that the majority of new 5e players have played some version of D&D before (or at least some similar RPG) and are going to bring those old assumptions with them regardless of how the PH words things. Maybe I am wrong; I haven't run a 5e game yet. But 5e being D&D, it is going to be difficult to shake off 38 years of historical precedent just because the PH is written a bit differently this time around.
Now for brand new players to 5e who have never played another tabletop RPG in their lives, I imagine those players will take the PH far more literally. I estimate they will be the minority, and not just the minority, but a small one, largely legacies who will already carry some of their parents' assumptions with them and video gamers drawn to the hobby because it shares the sword & sorcery theme. I do not see anything about 5e that tells me it is going to drastically increase the number of brand new first-time tabletop RPG players. The investment cost alone for just the PH is pretty high relatively speaking.
Some players will sit down to a 5e game having thoroughly read and digested WotC's modular concept and the PH rules themselves and recognize that "ask the DM" truly means "ask the DM." Even then, I imagine those will be uncommon cases, and having even one of those players in a party of four will be unlikely. Again, I haven't started up a 5e game yet, but using historical precedent, this is what I expect.
I think the best-case scenario is that after a few years 5e players learn to recognize that these things are truly options and not default rules, but the issue will continue to persist for players making the transition from other editions or games.
As I said, I could be wrong about all of this. But having read the PH and having had the experience I have had as a DM, I do not imagine I should expect anything different when I start a 5e game. As usual, I will presume the players presume the PH is pretty much all default rules and if things are otherwise the DM ought to say so in advance. I will be very surprised if I sit down to run my first 5e game and get questions like "are you using multiclassing?" from several players.
