Hiya.
Ditto on what Froggie said. I never left 1e. Been playing it more or less consistently for 33 years or so (since '81; started wit Basic in '79/'80). For us (my group and I), 1e allows us to pretty much play any style of game we want. We've done high-power (not really our thing), lots of mid-power (maybe 40%), and mostly low-power (60%). Favorite game worlds are Greyhawk (pre-wars) and Eisla (my home brew; very low-power).
The thing I think that draws folks to the OSR is, for me, the amount of planning a DM needs to do for "this weeks game". With OSR, the rules are *actually* seen as guidelines...they allow the DM and Players to make rulings on a case by case or campaign by campaign basis. OSR systems are, by default, very "modular"; it's dirt simple to rip out some aspect and replace it with something else, all without affecting much else, thusly allowing easy use of already published material. For example, take out the Saves of 1e and replace with a single "Save" stat; races/classes give bonuses to rolls (e.g. "Half-Orc's get +1 to Save against poison and disease"). Too simple? Ok, replace it with 3e/PF style saving throws.
The other major shinny factor is that OSR games have an assumed "player is the character" outlook. Basically, when a player is faced with "A 60' high stone wall blocks the way" the player is expected to ask questions for more detail..."The stone is rough granite, very solid, but not well made"...then the player comes up with a plan..."OK, Smergiopy the Stealthy will take the 50' rope and grapple and climb up to the top. Then he'll make sure it attaches firmly at the top so the others can try and climb up". That is opposed to more modernized games that shift the focus to the character, where you get more of "A 60' high stone wall blocks the way"..."OK. The thief uses Climb."..."So does my Mage"..."And my fighter, but I'll take my armor off first"..."Well, I guess my clumsy cleric will try too...oh hey! Climb is Str based! Sweet!"...
At any rate, I'm glad that the OSR occurred. There are a LOT more places on the net where I can ask "What page is the 'Detect Invisible by Intelligence' on again", and get a reply of "DMG, page 60" in stead of "Uh...what?".
^_^
Paul L. Ming