It isn't unreasonable for people to follow an expectation of receiving their device on April 24 when so much effort was initially put in by Apple to push that availability date. Ever since iPhone made Apple releases such a major thing, people have understood that with the inconvenience of showing up several hours early to wait in line, they could walk into their local Apple Store and walk out with their device on release day. Up until at least the iPhone 5, you could even reserve your phone online and do in-store pick-up so you didn't have to wait in line at the store, and could go about your day like normal, then wander into the store after you were done with work/school/whatever. You could also generally rely on two things from this setup:
1) You would have at least over half of release day to preorder your device and get it on release day, and
2) Apple would have the product actually available on release day to a majority of customers.
Those options weren't presented with Apple Watch. Anecdotal evidence suggests that any orders that weren't placed before 12:05am Pacific Time would have an estimated delivery date well past the heavily-advertised April 24 date. The online store was malfunctioning for many buyers, even among those who WERE trying to get their order submitted right at midnight, and by the time Apple got things working (which was fairly quickly, to their credit), people were being met with delivery estimates that were up to and over a month out from the heavily-advertised April 24 availability date.
At least with standing in line, buyers felt some element of control over their experience: if they didn't get a device, it was because they chose not to give up sleep or alter their sleep schedule (or take the day off from work) to go stand in line at the Apple Store. That option wasn't available to customers for the Watch, who were all told to go online to order, that Apple Stores wouldn't have any stock on release day.
So there's the first instance of frustration: loss of control over the process, and inability of Apple to meet their heavily-advertised availability date for enough customers that it functionally became not true.
Then, after Apple backpedaled and changed "Arrives 4/24" to "The Watch is coming", they started telling people that they were working hard to meet preorders, and would be fulfilling them in the order they were received. Except we have people on this and other forums indicating confirmation dates/times from their fulfilled orders that are AFTER others, and they've received their Watches while those earlier preorders are often not even out of the processing stage.
There's the second instance of frustration. Apple is offering no actual explanation for the obfuscated nature of their shipping on whatever stock they DO have. The anecdotal evidence points towards inconsistency with their public statement on how orders are fulfilled. Once again, some customers feel helpless (a feeling that could be greatly ameliorated with more transparency and response from Apple), and more than that, feel lied to, on both the availability date and on the nature of how orders are fulfilled.
It also isn't an uncommon retail expectation to believe that if you got your order in before someone else on most anything, that you would have your <insert thing> before that person. That isn't the case. It isn't worth fighting over (though some still do), but "line jumping" is never appreciated, and even if we take the statistically-insignificant Watch Edition out of the equation, we have people getting their Watch and Watch Sport devices meaningfully earlier than people who had ordered BEFORE them. It feels like digital line jumping, and the response from the retailer is utter silence.
And to top it all off, Apple has no social media presence at all: no Facebook page, no Twitter account. In the 21st Century, these have become vital methods of communicating with a company to express displeasure, questions, or even congratulations. Many companies like Comcast and other national chains have become much more responsive (and satisfactorily so) with customer service because they understand the power of social media, and the danger of real-time negative P.R. from dissatisfied customers.
But not Apple. Their general feedback form not only blatantly says to not expect a reply ("though we read every message", which few shoppers, seeing no evidence of that statement, actually accept at face value), but it has character limitations in its text fields that can in some cases prevent a customer from fully explaining their individual issue, such that they have to go through the frustration of explaining it in very slow time with an Apple customer service rep... when they can get in touch with one.
If I had to boil it all down, my belief isn't that people are merely unhappy about not yet having their Watch, but they're unhappy about the circumstances that led to that state. Apparent inconsistency with stated fulfillment priorities, obfuscation about what the actual hold-up is, and inconsistency with otherwise-common fulfillment practices (i.e., "first come, first served").
Lacking a more direct method of input to Apple, it thus comes as no surprise that this thread is over 120 pages (and growing) as people come to realize this is one of the few "official" forums where they have a small chance of actually being heard by a company whose practices are starting to communicate to some that they really don't care about the customer experience nearly as much as they say they do now, and had shown they did in the past.