Based on my experience with the play test, here are some items that I hope make the cut in the core books:
Backgrounds / Traits
In a nuthsell, Background and Traits add a veneer of backstory to a character and give the character some benefits in terms of related skills or bonuses. This was a cool idea to help players skin out a background for their PC quickly and easily. Some players will want to write their own back story novella with all original content... But, in my experience, many players (especially new roleplayers) want a basic character concept template that they can flesh out or customize. Backgrounds and Traits give just that. The playtest packet only had a few for each class. I'd like to see at least a half dozen different background templates for each class, if not more. I've always like the idea of PCs having had some kind of "job" before setting out on an adventuing career, even if it was only an apprenticeship to a leather tanner.
Based upon comments from Mearls and company, this will probably not make the cut, but In some versions of the play test, instead of getting a flat bonus (such as +2) for having a skill, you rolled an extra d6 and added it to your d20 roll. The designers seemed to believe this rule was too fiddly and created too much numeric variance during skill rolls, but I actually liked it. Because the play test packet changed skills to add only a flat +2, we house-ruled using a d4 instead of a d6 in my game. This allowed us to continue using skill dice, but kept the skill check math a little closer to the play test since a +6 is significantly higher than a +2, whereas a +4 is only a small extra bonus.
While Feats are not new the the game, the way they are now described and presented adds more roleplaying flavor and ties in nicely with Backgrounds and Traits. Instead of picking "Expertise (Bow)", "Precise Shot" and "Double Shot" over the course of several levels, you now select "Archery Master" which gives you several benefits at once. One of the new edition feats replaces about 3 of the previous edition feats. This means a smaller feat list and the elimination of Feat trees (along with the accompanying analysis paralysis). Feats are more powerful, but recieved less frequently, making them feel more special. The descriptions are also more evocative. The roleplaying enthusiasts can potentially run with this. Now my character doesn't have Bow Expertise, but rather he is an Archery Master. It's a subtle shift in presentation language, but really feels like it adds depth the the character background.
Advantage / Disadvantage
It was unclear if this mechanic would make the cut, but it appears very popular among the playtesters, so I would assume it will. Advantage allows the player to roll two d20's and take the higher results while Disadvantage forces the player to take the lower result. Statistically, this results roughly in a +/- 4.5 benefit or penalty to a die roll. The challenge I had with the mechanic was when to apply Advantage or Disadvantage. There were a few specific circumstances noted in the playtest rules, but it was hard to judge exactly when this level of penalty or benefit should be proferred.
In the playtest packet, we only got a glimpse of the Sorcerer utilizing spell points. Unfortunately, the class was over-powered and broken, but it hinted at the direction the D&D designers were heading. My biggest issue with using spell points in this way is that the PC still has to pick from a spell list. Instead of using a first level spell slot for Magic Missile (or whatever), the Sorcerer would use 2 spell points instead. This difference in mechanics means the Sorcerer is a little more flexible in the combination of spells she can cast during the day, but essentially, Socerer magic is indistinguishable from Wizard magic since they use the same spell list. Yawn.
It would be nice if a spell-point system could be radically different from the Vancian (memorize spell slots) system. I imagine (and am actively trying to work out) a system of improvised magic that uses spell point similar to Dragonlance 5th Age (more on this later). It would be great to see an option spell point system that had a similar flexibility, but I'm doubtful that will come anytime soon.
All in all, based on the playtest D&D appears to be shaping up pretty well and despite owning almost every prior edition, I'll probably suck it up and spend the money... again. There are still a few things that irritate me that should be fixed but probably won't (I'm looking at you Carrying Capacity and Equipment Weights) but that can wait for another post.