Sleeping Darkness:
Yeah. The only thing I can imagine is that prestige classes were done by a small team who had no access to any other team's work - you have all these classes with great scaling features and mix-and-max options, and then prestige classes that typically want you to sacrifice all of that for extremely niche features. It gets even worse when you consider archetypes - because archetypes basically do what prestige classes used to, provide you different advancement options to suit a particular concept and optimize your character.
Well, the first prestige classes were just the 3.5 DMG ones retooled for Pathfinder, and I agree, they were lackluster, even compared to the straight, no archetype basic classes in the Pathfinder Core Rulebook. And they had all the same problems of their 3.5 parents too: the assassin's special attack is way too easy to resist, getting into mystic theurge or arcane trickster means multiclassing in a way that leaves you behind the curve for 10 levels, and so on... still, some of them are okay.
But after that, if you really look, the prestige classes come from region books or theme books for the Golarion setting. So Osirion, Land of Pharaoh has a thematic prestige class thrown in, and Pirates of the Inner Sea has another, and so on... of course those are extremely thematic, niche prestige classes. The very book they come in pretty much require it. Excellent fluff, a niche mechanic... but if you step outside of that niche, the mechanic can't make up for all you lose. Unless you're playing a pirate, and the campaign takes place on a pirate ship, the Inner Sea Pirate (or Deep Sea Pirate on d20pfsrd.com ) just isn't attractive at all. There's no 'Ultimate Prestige Classes' or even 'Complete Adventurer' in Pathfinder, just a lot of setting books with a prestige class or two each.
Which, in fact, pretty much proves your point. Each prestige class was done by a team who, at least at the time, was only doing one prestige class; and was doing it in the context of a setting book, for fluff, not as part of a complete set meant to stand on its own.
The Dungeon Master:
I have always started my games at 1st level but over time I'm starting to see the appeal of 3rd or 4th. At 1st level everything is starting to feel same-y. The wizard casts sleep, the barbarian just hits things with a big stick and the rest of the party kind of muddles along mostly failing skill checks and attack rolls.
I'm thinking of having a sliding progression scale. So Fast progression from 1-3, Medium from 4-12, and then Slow for 12+ so the game doesn't run out of challenges too quick (although there is always the legendary Tucker's Kobolds:
I like playing spellcasters; sorcerers mostly. But a level 1 sorcerer, or wizard for that matter (it's worse for wizard), suffers terribly from one problem: they just don't have enough spells per day. Well, they have cantrips, but time and again in combat you find yourself falling back on your crossbow, or quarterstaff, or dagger... not to mention that half the spells in the book, even the level 1 spells, are in fact useless at level 1: a 1 round buff isn't really worth casting, most of the time. Or a 1 round summon...
It's okay on tabletop. You play one session, combat goes by quickly, the fighter wins the day, and then everyone advances to level 2 and soon enough the sorcerer or wizard is actually doing magical stuff most of the time. But in PbP, it will take months in real time to get there - if you ever do, and for most of that time the spellcaster will play as a very inferior fighter because he's either hoarding his few spells for when he really needs them, or doesn't have any that apply to the situation or has already cast them and is waiting impatiently to get them back.
Personally, in PbP, I recommend skipping xp altogether and just having the GM hand out new levels when he's ready. And preferably very early for level 1 characters.