It's not the existence of errata that ticks people off- mistakes will happen, after all- it's the amount and nature of errata in a supposedly professionally done product that is galling.
Why is this galling?
No matter how professionally you craft a product as complex as D&D, you're going to have
many facets of it that are deserving of change or adjustment in order to improve the game experience.
Given this, you can go one of two ways:
1. How things were done in the past; that is, infrequent errata released without significant explanation years after the original problems were identified.
2. How WotC handles things now; that is, frequent errata/updates with explanations released in a timely manner.
The first strategy makes things nice and simple, but fixes little. The second fixes everything that needs fixing, but requires that you keep up with the errata (which, honestly, is not difficult and usually goes something like "Hey, this particular rules item seems too powerful/weak, I should check to see if there's an updated version!"). To boot, DDI integrates errata as it's released, so if you're a subscriber you don't really have much to keep up with anyway.
I hate to do this, because I just know that there are one or two people who will read this who
actually believe that D&D 4e is like WoW on paper, but you don't need to look any further than WoW for a good example of what I'm talking about. WoW makes changes
constantly to ensure balance and fix bugs. It's almost guaranteed that you will see changes occur on a weekly basis, and when things are particularly hectic they will push hotfixes daily.
Does this reflect on Blizzard's professionalism? Absolutely -
in their favor. Regular and thoughtful updates make it clear that they care about the quality of their game, pay close attention to how it is played, and dedicate significant resources towards fixing any problems that crop up in a timely manner.
The idea that Blizzard can be lauded for acting as a responsible and dedicated shepherd of its game system, while WotC is derided for doing the same, is a little appalling.
Oh, and if you're coming from the standpoint of "They should have gotten these things right the first time around!", you're speaking from ignorance. It is not possible to develop an intricate, expansive game system (whether a tabletop roleplaying game, video game, or otherwise) without later discovering things that ought to be fixed,
especially after release when your effective crew of testers (your entire audience) begins to identify problems.