Swords & Wizardry probably has the biggest jump from modern-type gaming of all the retro-clones, just because it goes all the way back to 0e. There's one version,
Whitebox, for pure no-supplement 0e, and the
Core Rules use some material from the supplements, most importantly the variable weapon damage and hit dice. I happen to prefer the core rules because I'm a house-ruler and there's more room in the numbers when using the variable hit dice and damage. Many people prefer the WhiteBox because the variable hit dice of Supplement I (represented by the S&W Core Rules) are the game's first instance of "power creep." S&W allows the choice and contains the numbers for using it with either ascending or descending AC.
The other big jump into a new flavor and gaming style would be to go to one of the Moldvay Basic approaches, which are Solomoriah's Basic Fantasy Roleplaying Game (BFRPG) or Dan Proctor's Labyrinth Lord. These are highly similar, and actually compatible with Swords & Wizardry as well. Labyrinth Lord is more faithful to the Moldvay Rules, BFRPG has some tweaks to improve it (ascending AC, for example).
If you're a true fan of wonky subsystems, OSRIC is the way to go. Swords & Wizardry, LL, and BFRPG are all based on a highly free-form, free-wheeling style of gaming. OSRIC, as a clone of 1e, provides much more wonky "crunch," and far more detail than the 0e and Basic clones. If you want old-style gaming with a bit more guidance, and if you like getting inspiration from the nature of the game-rules themselves, OSRIC is probably the choice for you.
Hopefully that helps identify what sorts of gaming preferences are best facilitated by the various systems. WhiteBox has a gritty, risky, human feel to it; Swords & Wizardry Core Rules are very freewheeling but with more familiar numbers - still 0e, but with more "stuff"; the 2 Basic clones are also freewheeling with a bit less of a sword & sorcery feel than S&W but with their own pulpy flavor; 1e/OSRIC is the game for wonky subsystems and more rules for the DM to rely on instead of really free-form rulings.