Man, it's been a while since I did an out-of-character / rules post. Tim's been running a fun Trailblazer test campaign (which was to be a one shot, but then ran long). So, some thoughts on what we've seen so far from Trailblazer, in no particular order:
Fighters: nobody's gotten to use Punishing Strike yet, which adds attack and damage on AoOs :(. Tim fielded a pile of NPC fighters during the first session, and I've been running a fighter / ranger the entire time, and AoOs just don't get provoked for movement like they used to. Another consequence of the change to AoO rules is that combatants can now circle each other, rather than standing in place. A point for realism! Expert Weapon Proficiency, though, has been wonderful; I used it to make Dwarven Waraxe a light weapon, which means I can two-weapon fight with them without penalty, which is awesome (plus Weapon Finesse, being able to use it in grapple, and being able to use it to cut my way out of Swallow Whole attempts. Not that those last two have come up yet). Tim's fighters used it to boost damage, I believe, and their attacks stung quite a bit. So: fighters aren't complete chumps anymore. And Combat Reflexes is an awesome bonus feat, because it get you more...
Combat Reactions: Also awesome. These let you do other things with your "Attacks of opportunity per turn" pool. DR from Blocking saved my hide from the black dragon in session 2 (as did a potion of bear's endurance; lacking either, I'd've been toast), and Dodge, Aid Attack, and Aid Defense have all been used to good effect as well. AoO has been used the least frequently of any of them, really, which is a nice change. Having Dodge and Block scale with BaB also helps out the fighty-classes, which I appreciate a lot (and the extra Block DR from shield bonuses also makes Sword and Board a much more viable loadout).
Bard: Tim tasked me with building and running Adam, the party bard, in combat. I was a little saddened by this, since I'm usually not a fan of bards at all, but man... bard buffs are good stuff. I enjoyed being able to time Haste and Inspire Courage when it was most useful for Fjolkir, and really miss the damage bonus from Inspire Courage. On the plus side, the fact that Inspire Courage largely overrides Heroism in combat meant that I was able to claim all the Heroism potions, which is paying off now that Adam's gone. Also, Power Chord is a really nice ability; being able to swift-cast Cat's Grace and Glitterdust was useful several times. Overall, my respect for bards has improved significantly, though I don't think I would want to play one as my only character.
Ranger: About the only things they changed here that have mattered so far were opening up bonus feat selection (I used it to get Two-Weapon Defense since I already had TWF; wouldn't've been able to do that normally), Improved Reaction (+1 Init; since I also have Improved Init and 18 Dex, I usually go first. Nice.), and spellcasting. Getting low-level spells to be able to heal myself a little and to use Longstrider, even as a multiclassed character weighted mostly towards fighter has been really useful, but improving my spellcasting is slow going; I'm not going to get 2nd-level spells until 10th level. I'm currently at Fighter 6 / Ranger 3 - I think the plan out to 12th (not that I expect the game to run that long) is Fighter 6 / Ranger 4 / Fighter 2. I'll get 2nd-level spells from the next level of Ranger (as well as Uncanny Dodge and Speak with Animals at will), then Expert Weapon Proficiency from Fighter 7 for 2d6 damage on the waraxes, and then Greater TWF at Fighter 8. Which brings us to...
Iterative Attacks: TB changed iteratives so they work like TWF, Flurry, and Rapid Shot - extra attacks at a penalty to all of them. Being able to roll a handful of d20s all at the same bonus has sped my turns up nicely. Jared (who plays Somak) has also taken advantage of it, though Dan (who plays Alonso) really hasn't yet; he keeps rolling them sequentially rather than in parallel. I had to go buy a fourth d20 to roll all of my attacks, but it was quite a pretty die, and I don't regret it. Four attacks, each at -4, has been pretty effective; I'm very very likely to hit with one of them, and usually hit with two. It's basically the D&D application of the Starmada principle that "Volume of fire is important," and it's easy on the maths. I almost with I had Power Attack so I could really benefit from the math reduction this change saves.
Skills: Being able to scatter points across class skills and get effectively four ranks for one skill point has been useful; I've been spreading mine around to get a few points each in things like Swim and Heal (which actually helped recover from the ability damage from Malevolence last session). I think I'll probably get Knowledge (Nature) next level... something I really ought to have, but I bet Somak's got me beaten on it. Being able to assist wouldn't hurt, though. Getting 4+int skills as a fighter was also really useful; I'm at max ranks in Stealth, Survival, Perception, and Search, and then I got the class bonuses from Ranger. I'm as sneaky as the rogue, have the best perception in the party (thanks for the Eyes of the Eagle, Tim), and can track a party of four humanoids across firm ground a week and eight hours of rain after they passes. Haven't gotten to use search yet, though. Anyway, being good at useful skills as a fighter is a nice change of pace.
Druid: Somak's been effective in melee, tripping with a modified guisarme (using TB's melee weapon customization rules; he reduced crit threat range and boosted base damage) and Enlarge Person from Barridan, but he has a hell of a glass jaw; at last check, Somak at 9th level had around 36 HP, while Barridan, the halfling sorcerer, had 42ish. Probably poor HP rolls on Jared's part, but it puts the druid solidly in the second line of melee behind me (for reference, I'm at 85 HP. Toughness got upgraded pretty hard in TB, and Con 16 and d10 HP are awesome). The bonus to hit prone foes in melee has really helped offset my full attack penalties, especially now that Adam's Inspire is gone. Unlike the 3.5 druid, though, the TB druid hasn't been dominating melee in wild shape (they really toned Wildshape down; it's kind of like an Astral Construct of personal buffs now); he has been succeeding wildly on his trip attempts (with the size bonus from enlarge, using a two-handed weapon, ability score buffs, and ability score boosts from Hybrid Form), but he's not an unstoppable bear with an army of bears (no animal companions, and summoned creatures are fairly weak - the dragon slew a Summoned Monster IV from him in one shot of its breath weapon). I think we're all pretty happy with the changes here.
Rogue: I think Alonso's been less than maximally effective due to poor design choices by Dan, rather than by poor design by the TB folks. This is all.
Sorcerer and/or Cleric: Barradin was initially a Cleric 4 / Sorcerer 4, but eventually realized he was basically just using sorcerer spells and leaving curing to the wands, so Tim let him switch to full Sorc. And he's been throwing fireballs like a sorc ought to be; he picked up Empower Spell at 9th, and has been using it on his Scorching Rays and Fireballs to good effect. No complaints here, though I'm sad that we didn't get to see a proper half-and-half caster in full, versatile action (they operate very similarly to a caster of full level in each class, but lose out on some bonus slots and readies from each class). I'm also sad that we didn't have a proper caster / non-caster multiclass; I guess I'm the closest, with +2 BmB from my fighter levels and +1 BmB from my ranger levels. Maybe next campaign I'll run a Barbarian / Druid or a Cleric / Monk...
Not a whole lot else to report... we haven't run into any really annoying parts of the system yet, that I can recall. Oh, Dispel Magic. We're not sure if we like the Dispel change or not... they changed it so that it got to dispel at most one effect from the target, so that you weren't stripped of all of your buffs in one go. On the one hand, this is really nice for your PCs if they like to buff and your enemies like to dispel. On the other hand, Tim's NPCs have been better buffed than we have (except for the fight against the dragon, where we drank so many potions that if we had been using the old Potion Miscibility rules, we'd've all died), and his enemies haven't been using dispel, so we've been getting the short end of the Dispel Magic stick when we go to dispel their buffs. I guess the real problem is buffs themselves; they're annoying to keep track of, and Dispel Magic just means you have to recalculate everything in the middle of combat. Tim proposed changing their durations to encounters, rather than simulationist units of in-game time; this could be justified by the fact that if you're not actively using the benefits of the buff between combats, the power of the spell isn't being expended, or it's just leaking away slowly enough that it'll evaporate over a rest period, say. It's an interesting solution, but we haven't tried it yet. I almost favor removing buffs from the game wholesale; TB's spine analysis didn't take them into account and still came out which reasonable numbers, which suggests that they could be removed without severely breaking any of the core combat numbers for the game. Alternatively, granting inherent bonuses would work nicely as well; just give me something that I can write down and add up once, and then not have to remember. Fodder for another post on another day.