I am extremely skeptical that 1e or 2e were significantly easier in that regard. The main difference is one of expectations. Non-newbie DM knew that such things were error prone, and had no expectation that the something like a magic CR rating could be counted on as being useful.
I'm not sure if you're an old-timer or not, but, as I said, that's my experience. The idea that encounters were tough to balance seems a relatively recent issue. While expectations may be a part of it, I think they'd go more along the lines of "You should be smart enough to avoid putting a "can only be hit by +2 or better" monster up against your low-magic party." Rather than "Its just a lot of work to DM and spend a lot of time carefully balancing these encounters."
Hey, one night your low-level PC happens to run into a vampire because that is what the random city encounter table says. These things happen. Time to roll up a new character.
ACs went up. THAC0 went up. Damage went up. Weird effects went up. DR scaled up brutally fast. SR scaled up brutally fast. Saving Throws did not scale up, so much.
It is not very different, overall.
You're correct to an extent, but its not the question at hand. Specifically;
If your party finds a magic item, and magic items are not part of the fundamental math expectations of the game, how do you prepare balanced encounters? (or how difficult is it?)
So, yes, if you were using a random encounter table back in the day, you'd expect a lot of low-level character death. However, that was an expectation of the game from the get go, anyway. In any edition, fights that are either cake-walks or nigh-impossibly deadly are possible (although, likely stretching the rules in 4e.) The real question is, how likely are you (as a DM) to whip one out unintentionally, or how hard is it to make sure that your encounters are in that "sweet spot"?
You're right (except ThAC0 went
down) about the characters getting more competent and the monsters getting more competent as well. However, somehow, it just wasn't that hard to eyeball it and make interesting encounters...I'm not really sure why or what mechanics I'd point to, except that 3e was the first edition, AFAIK, that was written and designed with such a window in mind. I've heard old-timers argue for all sorts of things being the case: Saving Throws, HP, changes to monster mechanics, the general power curve being gentler etc. all have their proponents. ::shrug:: Maybe its just some weird gestalt thing about the entire mechanical system.