Good news is there's effectively zero randomness in progression for most characters*, beyond rolling for hit points. And you can even house rule that out if you want. So all you need to do is look at the races/classes in the book and their progression charts and you'll know what to expect.
_* The exception is, of course, wizards and elves. When they advance they gain new spells. If they encountered spells they can hang onto, captured spell book or scroll or some other means, then they can choose to learn that spell. Otherwise it's a random roll what spells they get. There's also mercurial magic, which wizards and elves roll for all their spells. This makes spells and magic far more interesting than most other magic systems.
I've found progression to be just right for long-term play. Not too steep and not too shallow.
But, like with early D&D, progression is mostly based on the Judge giving out magic items and the PCs earning diegetic rewards. Typically through Questing For It. Which is honestly phenomenal and should be stolen and ported into every RPG ever.
The main book has 60 pages of monsters, which is about 80+ monsters. The stat blocks are a paragraph. Plus they give, typically, several paragraphs of description for each. Though some are shorter. The selection of monsters includes several broad categories like dragons, demons, ghosts, and slimes. Each of these is unique in that you pick or roll from charts. The dragons, for example, have at least 293 million unique combinations. And that's with rounding down and skipping a few things. Converting monsters from most D&D editions is a snap.
Dungeon Denizens, the dedicated monster book, has over 500 monsters. It was funded on Kickstarter and is nearing delivery. It should be generally available soon. Though there are also too many zines and free resources to count.
The DCC modules are all high quality, almost without exception. Each has unique monsters, puzzles, and traps you can use regardless of running that module or not. Most modules are short and cost $10-15 in print. And no, despite the name, not all the modules are dungeoncrawls. Most can be run in a few sessions of play or stripped for parts. Converting modules from most D&D editions is a snap.
Goodman Games recently crowdfunded Grimtooth's Old-School Traps for both 5E and DCC. The PDFs were just delivered to backers so the books should be generally available soon. Converting traps from most D&D editions is a snap.
Despite the name, Dungeon Crawl Classics RPG can do all the same stuff any fantasy RPG can do.
It does have a distinctly old-school feel. Mechanically and fictionally. The game was designed around Appendix N fiction and replicating that style of action-adventure. So, just like D&D, most of the rules are about combat. But, just like D&D, you can do anything you want with the system. Two of the licensed settings are Vance's Dying Earth and Leiber's Lankhmar.
DCC RPG explicitly trusts the Judge to run the game and the players should trust the Judge. If the players want courtly intrigue, awesome, run it. If the players want wilderness exploration, awesome, run it. If the players want cozy shopkeepers, awesome, run it. But it doesn't have mountains of subsystems covering everything. It has the rules it needs to be a recognizable d20 fantasy game then gets out of the Judge's way, like it should. This is intentional. Though there are plenty of fan-made resources available free if you feel you absolutely have to have mechanics for some of these things.
You can Quest For It to gain new skills, new allies, magic spells, magic items, castles, keeps, etc. Basically, all the customization questions people typically have come down to Quest For It. You as the Judge put whatever reward the PC wants on the far side of a compelling quest and set them loose. Anyone with even a basic sense of story and conflict should be set.
Most of the question people have about the game come down to either "The Judge is in charge, they decide" or "Quest For It."
Post #3 of
this thread has a gathering of resources if you're interested.