OK, so I've got the play test, read it and played it. I actually like much of it, including the Backgrounds and Themes ideas. It generally feels like D&D (as an 'old school gamer') although it still needs expanding on some ideas.
The big bugbear for me, however, largely boils down to to same core issue I have with D&D 4th edition (and Pathfinder actually). And that is the power creep.
Why do Hit Points have to be so high at 1st level - out of synch with all other NPC dwellers? Why do Fighters need to have a D12 Hit Dice, and cause unstoppable damage at 1st level? Why do Wizards need an inexhaustible supply of unerring Magic Missiles to launch? Why do classes all need a schtick-like effects to be enjoyable to play in the 1st level?
Seriously, if your group wants to play a higher level style fantasy, what is stopping them simply giving all PCs 10,000 XP to start with and simply begin play at a higher level?
For me half the fun of D&D is starting off as relatively ordinary characters, and then becoming heroic as they gain experience. This seems to be lost in practically all modern iterations of D&D. I recall the 1st time I played D&D, being wowed by the experience of our party overcoming an Ogre with something like 30HP at the climax of the session. You lose that sense of danger, if the individual characters have nearly that much HP each already. You cannot play a Lord of the Rings style Hobbity adventure with these rules, which is a clear failure in my book.
So, for me:
1) Make the HD the sole measure of HP (with a Con modifier for each level). Have characters gain up to 10HD at 10th Level, then simply stop awarding them after that.
2) Give Fighters a d10 HD again. Actually, I'd arguably give them a D8, so that the HP are equitable with other characters and NPC Warriors). Levy their 'Feats' so that, at 1st level at least, combat is challenging. They can gain more dramatic feats as they progress, but it needs to be levied.
3) Make Wizards 'minor spells' actually minor in effect. Anything that directly causes damage, without needing to roll, is not a minor effect. Cantrips should be effects that gain useful little benefits, like opening doors or moving small objects around, but are not flashy evocations of power.
4) Make the skills the main focus of the Rogue Class - not just the 'striker' role (although, admittedly, this is much better in D&D Next than it was in 4th Ed). I'm not asking for big long lists (definitely not!), but what about being able to pickpocket again?
5) Be wary of escalating bonuses. Already, at 1st level the Fighter seems to have massive bonuses on damage and attacks - indeed, almost all the characters have bonuses of some type, and it's hard to track where some of them are coming from. Also, incidentally, are they going to go back to adding 1/2 Level to Skill checks and Attacks? It is not clear in the play test, although I actually wouldn't mind as it's an easy method of calculating.
