Q:

Have a problem? Toss money at it and it will go away!

The title is a bit bombastic but in my opinion it fits the trend I noticed here and which I think needs to be discussed. 
To me it seems that many users here do not understand that some other users may have different design constraints than they do. Comments to this question made me start analyzing the problem. OP here complains that he can't find a two-pin phototransistor and one of our users claims that they are available at Digikey and believes that if OP can't get parts from Digikey, the question may be too localized. 
Well, OK, that's one way to look at it. On the other hand, I believe that such attitude is simply bad engineering practice. In my opinion, one of the main objectives of an engineer is to make a working product with available resources, if that is possible.
Another interesting example of this trend are comments to this question. Again users can't accept that OP can't just buy a better part.
Yet another similar comment thread can be seen in this question where again some users can't understand that sometimes you just have to use datasheetless parts if you want to be competitive.
I have an example to post too:
Once I was recommended on this site to use 10 µF ceramic capacitors at input of voltage regulators. And where do we go to get them? Digikey of course! They're a big huge distributor and they have millions of articles in stock. The capacitors I linked look interesting and not too expensive so I'd probably buy 50 or maybe even a 100 of them and be stocked for a while.  That's $14 for a 100 pieces. 
So let's take a look at Digikey's website for my country. In the middle o the screen, the second center picture from top proudly proclaims that the transport price from US to Serbia is fixed at $120. Well my $14 order just became $134 order and each capacitor now costs me $1.34 and that's before customs and forwarding fees. For $1.34, I could buy an entire simple voltage regulator here and more. If I take local administration into account, the single capacitor would be more like $2.5 to $3. For $2 I could get a PCB with a full-wave rectifier bridge, needed passive components and a voltage regulator. This disqualifies the capacitor even at optimistic price of $1.34 a piece as an economical solution to the problem.
Of course, the solution to this example would be to look for another distributor, but my idea in this post isn't to solve this particular problem but to start a discussion about the trend I've noticed. 
What I instead believe is that we should try to form some sort of a guideline when it is acceptable to recommend a solution of selecting a new part and when we should believe OP that a better part can't be found.
So any thoughts about this?

A:

I know this is a personal issue and it looks like all the "damn lazy Americans" just want to buy premium parts from Digikey, but hear me out.
I don't have any problem with specific questions about specific parts, but it's really frustrating to have the OP say "Nice answer, but I can't use it because I actually have this other (significant) constraint that I forgot to mention."
Here's the thing - a lot of these questions don't have enough information to give an answer that would benefit the OP or even the community in general. This leads to a lot of people trying to guess what the real question is, and are trying to make a bunch of assumptions. There are problems with this setup:

We don't know if the OP has actually purchased a part, or is just planning on buying the part they mention. If they haven't bought a less desirable part, then perhaps they would find it useful to know that there may be better alternatives.
We don't know why the OP has chosen the part. Did they choose that TL084 because it was what they used in college, or because that's all they could find?
We don't always know where the OP is asking their question. Is their selection limited because they are in a foreign country or because they don't know where to look or what to look for?
We don't know whether or not their selection is limited. For example, you could probably get manufacturing quantities and prices on the vast majority of parts almost anywhere, but we don't know if the OP only has access to local hobby stores.

So now on to the problem that this causes - multiple answers for the same question, where users will pick the correct answer based on where they are located or what resources they have available, a.k.a. localization. This is probably the inevitable result of questions with a shopping component. I don't think we should get rid of these types of questions, but perhaps we should require more information up front. There isn't really an easy answer to this problem, which is probably why these types of questions are forbidden on other StackExchange sites.
If I want to help the poster, should I withhold my opinions because I don't know where the poster lives? Should I withhold my answers if I don't know what parts are available at the electronics stores in Belgrade?
For your op-amp example, I might recommend using an op-amp with a better offset voltage rating than the originally suggested TL084. If I compare the increased cost of the better op-amp to an offset compensation scheme that would be needed with the TL084, then I can find a cheaper solution with better performance. For manufacturing, no trimming of the input stage would be a huge plus, and I can even win in total price when I buy them as singles.
This answer would be completely inapplicable to your situation, if you can't get a decent/similar selection of op-amps. The cost of a trim pot could be significantly less than the cost of acquiring a better amplifier, in terms of travel costs or shipping/handling costs.

A:

When it is acceptable to recommend a new part?

Unless the OP specifically states that they cannot change the parts used in the design, it is always acceptable to suggest any solution - including a new part - that will solve their problem.
When the OP explicitly states that they have to use a specific part, any answer that suggests a different part isn't ideal, but it's not a bad answer, especially since we're building a knowledge base for all engineers to visit in the future.  IT may be that the "ideal" solution of switching a part doesn't solve the OP's problem, but it may make them more carefully consider their requirements, and it will help out future users with a very similar problem but who do not have that constraint.
The OP doesn't have to accept that answer, and can leave a comment indicating the poor fit for their particular application.
There is no need to flag, delete, or complain about answers that might be useful for future searchers.
So far your examples seem to show that the system of voting and comments is working.  All except the altium footprint question have accepted answers, and the altium question won't be magically answered if we tell people they can't suggest a particular course of action, so solving this problem won't affect whether that question is resolved or not.
Generally, I'm opposed to restricting what types of answers can be provided unless the answer shows clear abuse of the system or misleads the OP.  Engineering includes the ability to look critically at the requirements and question their necessity at times in order to improve the possible solution set.  Restricting the answerers will only lead to fewer people willing to spend their time answering questions.

A:

I would like to encourage everyone to stay playful. For instance, I work for a big company that produces yet-unheard-of top-notch products and during development, we get our parts as samples or from distributors; when choosing them, purchasing politics often play a bigger role than cost.
However,
the majority of my most entertaining and educating hacking happens at home when I play with rules like obliging myself to the design constraint of using parts that I exclusively de-solder from junk boards found in my cardboard boxes.
Top secret info: My work involves safety circuits that need to behave in certain ways once a part fails, and another part fails. This stuff is not mass-market (not economical for IC manufacturers), and sometimes, the only solution involves 2N3904/2N3906 transistors with some additional passive components. Guess why they didn't fire me during the bad economy in 2008/2009? (Hint: Some discrete-component magic next to high-end FPGAs!)

