You have to pick:

This would have some drastic effects on climate. The Tibetan plateau is currently about 3500 meters above sea level, so every currently-inhabited place on earth would be at least that high. Thin air, cold temperatures, and reduced precipitation would wipe out all the current agriculture going on up there, we'd have to set up new farmland on the exposed ocean floor as quickly as possible. If the silt down there is still salty civilization would probably be doomed and we'd have to start over from small enclaves of survivors (the abandoned ruins on the continental plateaus would probably be well preserved, a neat post-apocalyptic setting).

According to a bit of websearching I just did, the highest altitude where there's permanent human habitation is a town in Peru at 5100 meters above sea level. But above 3700 meters low oxygen levels starts to dramatically increase the risk of miscarriage, so there's another reason to abandon the continental uplands.

If, on the other hand, more atmosphere is magically added as the ocean drains to maintain the current air pressure on the continents, we get a new sea-level pressure of about 1.5 atmospheres. I don't know of any studies that have examined the effects of long-term habitation under those conditions, but I don't think it'd be bad. Could even be beneficial. The continental uplands would still become a lot more arid in this scenario, though, so we're not out of the woods yet.

@bryan.derkson But the sea level is dropping, the land mass isn't rising, meaning air consistency would remain the same.

@hellraezer

As the ocean level drops it will leave empty space below sea level where there used to be water. The air will sink down to fill it. So even though the land isn't rising it'll still seem like it is as far as air pressure is concerned and measuring altitude relative to the new sea level will still give similar results.