The errata .pdf has a note mentioning that it is designed to speed up play because fewer dice are rolled
I've found this to be a very, very silly justification for errata. FFG-WH40K is a pretty dice-light system (even if it is rules-heavy). You have percentile and, at most, like 10d10 dice (typically for high-end psy powers; the next closest is probably lascannon or vehicle weapon damage). Rolling 6d10 is NOTHING compared to, say, Exalted or Scion or Shadowrun, where you're easily rolling 20+ dice with great frequency. Really, the point of reducing weapons from 2d10 to 1d10 rolled damage is to lessen the occurrence of Righteous Fury. Obviously the more d10s you roll, the greater overall chance of rolling 1 or more 10s.
I agree that some of the weapons (bolters) needed a nerf, though I dispute certain aspects of the extent of the nerfing, mainly because bolters have always been portrayed as full-auto assault rifles and now for whatever reason they're semi-auto. I see the balance issue but I think it could have been better resolved with the long-existing fluff. Was S/2/4 really that bad, if you reduced overall damage done?
By the same token, I think the heavy bolter RoF nerf was a little heavy-handed (especially without using Black Crusade autofire rules) because now it's significantly easier to hit max RoF on big targets or Hordes; maybe less overall damage, but IMO also sort of unsatisfying. Pre-errata Devastators actually get something out of investing in BS-improving wargear; 6 DoS pops up pretty commonly though, and the ones I've played with always got a real surge of enjoyment out of hitting 8, 9, or somehow 10 DoS. It was rare enough to be a really cool moment. IMO, 6 is not near as news-worthy.
I think that plasmas and meltas needed a lot of help to be competitive under the core rules, but I've seen better melta ideas on these forums than the "double Pen" errata to the weapon quality, and how just doubling Pen actually doesn't help much against high-end vehicles like Land Raiders, which meltas are supposedly designed to counter. Plasma still seems a little weak and Volatile is a worthless Quality when fighting 2/3 of the most-commonly faced/major enemies in the Reach: xenos (because it's fully obviated by Deathwatch Training). It's only worth anything if you're fighting Chaos and even then...again, I've seen houserules that like more.
I find it funny that Astartes Assault Shotguns are now even better than bolters at close range, especially with the right ammo (read: the kind that gives Felling). Still unsure about the errata to lightning claws and whether they lost the special rule from the Core.
Do agree that Tau baseline weapons being better than Imperial ones is good from a fluff perspective.
I'm not sure if most fights are significantly harder. Special ammo still owns if properly used, especially vs. tyranids (hellfire) or hordes (metal storm). Combat might just take longer overall and expend more resources because of RoF reduction, which leads to fewer hits per turn against hordes (the main time-consuming enemies to kill).