It's a pretty tough argument, and players throughout the thread have weighed with their thoughts on the matter. This is an issue that has been apparent since the launch of Wrath of the Lich King, where many armor pieces have largely shared the same models. Take, for example, the Spaulders of the Giant Lords, a plate shoulder piece purchased from the Sons of Hodir at Revered reputation. It shares the same model as 22 other rare plate items, from drops to crafted pieces. That's a lot of similar-looking gear. This design direction ensures that players will find an easier time finding non-set pieces that match instead of looking like a clown. This was one of the criticisms hurled at the leveling items in The Burning Crusade, and Blizzard seems to avoided it in Wrath.
On the other hand, some players argue that Blizzard might have overdone it, with one poster diligently listing many items which share similar models. He laments the fact that he has had the same looking helm over half the time he was leveling up and for some time after hitting Level 80. As cool as the Death Knight armor looks coming out of the starting area, it was something of a surprise to find that the non-tier plate items in Naxxramas and Heroic dungeons shared the same model. If you took mostly non-tier plate epics at Level 80, you would look almost identical to a Death Knight coming out of the Eastern Plaguelands.
This time around, in Ulduar, there doesn't appear to be a non-tier "set" as many pieces (as data-mined by MMO Champion) appear to be recolored versions of Tier 8, which the original poster decries as "effectively draining every shred of fun and sense of accomplishment out of raiding as a whole." I'm not so sure about that, but melodrama aside, it prompts Blizzard Community Manager Crygil to respond, stating that it's a tough balancing act but by using this method Blizzard is able to "maintain a good solid well polished look."
To be fair, this isn't the first time they've done this. Many tier items in The Burning Crusade had recolored versions, allowing players to mix and match non-tier pieces with their sets. With some luck, it was possible to obtain a complete, yet differently colored Tier 5 look using various loose pieces. Unless most of the items revealed so far from Ulduar are mere placeholders, it looks like Blizzard has followed through with this design direction. Crygil admits as much, noting how Blizzard aimed to avoid mismatched gear, and that this is how they choose to address the problem "for the time being."
While I do think that the base models are quite sparse this time around, I'm quite certain that there would've been whining either way. I think Tier 8 is very well-designed, and the current itemization means players won't struggle too hard to maintain the same, cohesive look (color discrepancies notwithstanding). That's certainly a good thing. On the other hand, players can't put together a completely different look should they wish to. To a degree, the original poster has a point about how every other person in Dalaran looks the same.
Personally, I will appreciate the flexibility of being able to keep just two or four pieces of a tier set for the bonus yet be able to maintain a cohesive appearance throughout. Should Ulduar itemization make it to live in this state, I wouldn't be so worried. This is the first time any raid dungeon will contain completely visually cohesive pieces, it's an experiment I'm willing to participate in. Blizzard has been known to change item models through patches, anyway, so even though it's a remote possibility, it can still happen with non-set pieces.