The first point I want to address is class balance. I don't think it's any secret to anyone that the druid and cleric classes in particular are really powerful compared to the others, and that some other classes, notably the fighter and possibly the rogue, tend to lag behind in usefulness compared to many others. This is quantified numerically (although the numerical values given to class features are somewhat subjective themselves) and pans out in accordance with conventional wisdom.
Then, to better balance the classes, they're all slightly rewritten. The fighter and the rogue, for example, are bolstered in several ways and most of the other classes were also rejiggered slightly. Some of this rebalancing wasn't just for balance; it also addressed some problematic, or at least finicky details about class abilities. Let me mention brief fixes to the Rogue and Fighter class (and a few combat house rules that also benefit them) as examples; I think I'm going to go about officially incorporating these from now on.
- Increase to d8 hit dice.
- Critical hits immunity of undead and constructs is removed. This means that Rogues can now Sneak Attack them.
- A "Combat Tactics" To Hit bonus that scales with level. This means that when sneak attacking, Rogue's attack with an effective BAB similar to that of a Fighter.
- Also, the consolidation of some skills means that the Rogue's skill points per level can go further.
- Increase skill points per level to 4. Also, see above on skill consolidation.
- A "Punitive Strike" ability that gives a major to hit and damage bonus on all Attacks of Opportunity (although conditions which spark AoOs are reduced in the Trailblazer ruleset)
- Additional Weapon Proficiency abilities that make the Fighter more able to do interesting things with their weapon of choice. This is an a la carte menu that includes such abilities as increasing your threat range, your damage dice, your reach, etc. with your weapon.
Some of the other classes have been slightly tweaked as well, and the way spellcasting works has been significantly overhauled. In general, I don't know that I agree that such radical changes were needed, at least for my games, because the highly broken classes were spellcasting classes (cleric and druid being the worst offenders) and I tend to de-emphasize them significantly. For example, if I allow them at all, I like to have them act as Advanced classes, with requirements to enter that place them beyond the reach of extremely low level characters, and only have ten levels that you can take anyway. With this, a lot of the balancing isn't as strictly necessary, although in the case of the Rogue and Fighter, I think they're kinda nice anyway, and they certainly go a long way towards making the classes more attractive to play.