the "standard" rules. Before I get into that, I want to point out (in case you somehow missed the news) that Wizards of the Coast has apparently begun offering pdfs from every edition on DriveThruRPG. There are currently some freebies, like B1: In Search of the Unknown and H1: Keep on the Shadowfell, though I do not know if those are temporary or not.
As a quick refresher on the character basics, players build them by rolling stats, picking a race and class, and...that is it. The standard rules, on the other hand, ratchet up the complexity to something more akin to 3rd Edition: skills and feats are a thing, and you can break out of the more traditional concepts. He uses the cleric as an example, stating that a basic cleric would use a mace and turn undead, while a standard one might worship Thor, wield a warhammer, and blast foes with lightning.
This sounds great in theory, as in my group some players prefer something simple and straightforward, while others enjoy trying to break the mold or even "power-game". Tastes can change over time as a player becomes more familiar with the rules, or sometimes you are doing a one-shot and putting a lot of effort into construction a character is just not worth it. One issue that comes to mind is the standard characters being more powerful than their basic incarnations (and, by extension, advanced being better than the rest). While I am not assuming that it will not generally be the case, I think that over time optimizers will find potent combinations.
3rd Edition style multiclassing and prestige classes are back in. While I like the system because of its flexibility, there were plenty of issues with it, specifically with class-dipping (where you take one or two levels of a class for the front-loaded features), spontaneous feature explosion (where you suddenly learn a bunch of stuff, like every 0-level spell and a bunch of 1st-level spells over night), underpowered or non-viable character combinations (fighter/wizards), or classes that only exist as vehicles for better classes (fighter, sorcerer, wizard, etc).
I think they are at least aware of the first issue, what with the "When you create a character whose first class is [name], you gain these benefits" clause in the Classes document. Maybe picking up a new class gives you a different or reduced starting suite? Maybe you gradually pick up everything over a slightly more lengthy period of time? Maybe a specialty allows you to nab more or everything, similar to 4th Edition's hybrid classes and the Hybrid Talent feat?
Actually with the exception of all feat cost, I mostly preferred how 4th Edition handled multiclassing: you spent a feat to gain just a bit of another class, and could gain more bits over time if you spend more feats. It handled the second issue, where a fighter with no magical background or training just suddenly got a spellbook with every cantrip and a bunch of 1st-level spells (or, conversely, a wizard was able to suddenly use most weapons and every form of armor).
It is nice, at the least, to see Mearls admitting to the third issue that can arise from all of this (the first of many challenges listed further down), so maybe this time around public playtesting will help weed it out, or at least mitigate it. I do think that it is important to note that not just power-gamers like breaking down a character into component parts: players interesting in a very specific concept or the narrative will almost certainly get some enjoyment out of building a character bit by bit.
For Dungeon Masters, the standard set of rules lets you create monsters 4th Edition style, using a set of damage, hit points, defenses, etc benchmarks. Given my past experience with 3rd Edition, this is great news as I found that 4th Edition made it very easy to create monsters on the fly. There will also more detailed rules added to the core for stuff like wrestling, swimming, etc. The breakdown is that the basic rules are for DM's who prefer to improvise, while the standard set is for DM's that like to tinker.
This could work out really well for people that want to start DMing, as several players in my group have tried but can get bogged down with too many rules and numbers. Even better is that you will apparently be able to mix and match rules complexity, so newer DMs (or those that prefer a looser game) can stick with the basics, while players can customize every facet of their characters, and vice versa.
Finally, Mearls lists a bunch of challenges that include the aforementioned multiclassing balance, as well as trying to ensure relative balance between options (including casters and non-casters), permitting some degree of optimization without allowing a massive gap, and allowing players to create characters based on a concept or story without making them too weak, as opposed to picking what you are "supposed" to. The only really odd "challenge" on the list was expanding the roster beyond four races...that sounds like it should be pretty easy, so hopefully we will see new stuff with the next packet.
Speaking of new stuff in the new packet, it looks like that the mystery class is the barbarian. Not much is mentioned, but apparently it differentiate itself from the fighter with a power-boosting rage and a more reckless fighting style. All things to be expected, but I am curious to see how it is conveyed in the mechanics. Personally I felt that the 3rd Edition barbarian was fairly similar to the fighter, while 4th Edition's take had clear mechanics that made them act and play much, much differently. I especially liked the berserker subclass, which could act like a fighter sometimes, but then go into a frenzy as the situation demanded.
The bit at the end about customizing your rage really excites me: the idea of being able to choose whether to stick with more mundane brute strength effects, or change your shape sounds awesome (especially since both a bear and earth elemental are mentioned). Absorbing the warden into a barbarian type works for me.