There is an excellent reply by Galadrin which focuses on the subsystems in B/X that really help improve character survival. These are things I have mentioned a number of times here but I liked the way that he put it.
PC's actually have a very high survival probability, when you think about it. Here is my argument, with page citation in bold.
1) If you think about it, by the book, every 1st level character except Magic-Users and Thieves is running around with AC 2 (Plate Mail and Shield is only 70 of the average 110 gp for starting characters, B12). Assuming the average party size of 7 (B19), that means 5 of the 7 characters are only hit 20% of the time (and with an average 5-6 hit points, thanks to the last line of B6, they need to be hit twice on average to inflict a casualty). That means a character must be attacked 10 times (on average) to kill him.
2) Combine this with the caveat that allows a Magic-User to pick his first spell (B16), very likely "Sleep", and you have a pretty tough party.
3) But who needs Sleep, when monsters only attack 28% of the time (B24)?
4) Even when a fight does break out, monsters tend to flee when they take their first casualty (B27).
5) All of these factors make it more likely that you can snag the treasure without a hard slog for it. The lair treasure of 24 Hobgoblins is already more than a quarter of a level for that group of 7, with even more for unguarded treasure, monster XP and so on.