In 3.5 revision, some of the changes are considered necessary. At least most of the updated spells were already considered in need of a fix by quite a large part of the playing community. IMHO this is a welcome update, but it sounds like WotC had a little part in it, beside simply listening to the house rules already used widely. Of course, everyone had its own house rule for Haste or Harm, and the results in the revision may reflect mine or yours. Anyway, these kind of changes needed just a reprint of the corebooks, which finally should have been almost errata-free.
But instead, WotC wanted (needed or whatever) to make a revision which would have been worth to buy. That's when they started putting down a lot of what dcollins calls "unnecessary rules changes". Some people like them, some people don't like them. The majority like them, except that you have to take into account the simple effect that anything "new" always have on the customers. As how they are really implemented, I don't think they make my game better or worse: damage reduction? I didn't like that before it was just a matter to get the next level weapon otherwise soon the monsters are out for you, but the new one although much more stylish it may just make you buy more handy haversacks for the golf bag (I was disappointed by the fact that there are too few materials: would they have been 10 times as many, probably you would just give up the golf bag idea); weapon system? I think the previous was easier, the newer more complete, but I won't often need other than medium or small weapons after all. To put together variants, there is a book called Unearthed Arcana scheduled for next year that would have been perfect for this kind of rules, with no need for 3 extra products to buy.
In-between there are changes like Power Attacks or Spell Focus. They were fine before, they are not horrible now either, they are just differently and not even completely. Except by giving the players the feel they have bought something new, they didn't need to be corrected to "fix the game".
Obiously, I am talking as a customer. If I was an executive in WotC I would be talking from his point of view. But I am a customer, therefore I talk as a customer. Never heard anyone being happy by the gas station increasing the price of gas because "it makes the gas industry goes better, for the good of all us drivers"...
But let's say something positive. Monster creation/advancement rules are the only structural change. 3.0 already shortened a lot the distance between a PC race and any other creatures, and 3.5 completes the process. IMHO this is the best thing coming to D&D since a very long time, and probably it makes the MM the only book which was worth a real new edition.
A word on magic items. The idea below standardizing magic items is good if seen as a tool for the DM, which indeed should be. The sad thing is that it turns into a tool for players, just as prestige classes, feats and spells found in non-core books. I think that every DM has the right (and he should be exercising it!) for total control over the topic of magic items in his campaign. But if rules have improved over time, many players have become used with the computer games way of playing, and tend to see PCs as going to supermarkets to buy abilities and equipment, after all the PCs themselves are expendable with the fast speed of advancement given by 3ed XP rules. Anyway, it's right that this has almost nothing to do with the revision.
A final word on the playtesters credit. I don't think dcollins is merely regretting the lack of credits on the books. If a company writes a gamebook which value resides in balance, playability or more modestly the fun it can provide, playtesting certification provides the proof that there was a lot below (during and after) the design of the rules. I work in the biggest mobile phone industry in the world and the hell they are selling phones without testing, and you don't know how many, extensive, and extremely professionally they are: in our case, the test reports may not be intended for the customers, but rather networks operators, consumers's association and similar, but in the case of D&D it's only the players. I really think that if they had been playtesting well 3.5 they would have all the reasons in the world to spend a page and put credits for it, but nowadays we are left with the spare words in the interview. I can't say they haven't playtested, but I can't say they have either.