First of all, let's do an extreme example:
Imagine that I post a question about a PHP Code Review "shield" and a C# Sudoku solver. Not as two separate questions, but as a single question. Such a question would be on-topic, it's working code, it is real code, and so on. But what does a PHP Code Review shield have to do with a C# Sudoku solver!? Absolutely nothing! This would be too broad. Different languages, completely different purposes - clearly too broad.
Okay, so let's see what we think about posting a question in the same language. Let's take a Poker Hand Evaluation and put that into the same question as a Tool for posting Code Review questions, both questions written in Java. There is still the problem about these two being completely separate things, they have nothing in common besides being written in the same language. They should still be separate questions.
Single Responsibility Principle
Now, let's take a question about Finding the sub-array with the maximum sum and a question about Minimum triangle path. Still, those questions have nothing in common. It's not one program for both, it's two programs. Could I write a program that contains both? Sure, I could. And if I would, it would be an acceptable question as then the programs would have something in common: They would both be accessible from the same class that I would have written.
As I said in chat, it's about whether or not you can say "You are violating Single Responsibility Principle" or not. If I would write a single program for both finding a sub-array with the maximum sum, and a minimum triangle path, then you could tell me that this program is doing too much, and I should put these things in separate programs and not try to tie them together.
Describing your code
It is also about being able to describe what purpose your code has. When writing a Code Review question, it is your responsibility to describe what purpose the code has, if you cannot make a reasonable single description for your code, then it is too broad.
So what do I think about some specific questions?
EBrown's question about project euler 1-5 - includes a wrapper to access all parts of the code, which means that the code has a common thread (all being accessible from one place), and "Project Euler problems #1-5" is a reasonable description. In my opinion, this is okay and not too broad. Saying "You should split these up into five totally different programs instead of trying to tie them together with some small piece of code" is a valid review.
Zak's question about 19 utility methods in VBA - No code provided to tie it all together. No reasonable description provided, but instead 19 different descriptions, each providing information about what one method does. Note that some methods are about arrays and others are about open and closed workbooks. In my opinion this is definitely too broad.
Note: Sorry for mainly using my own questions as examples. Those were the first one I could think of and the ones most easily accessible to me. This is not meant to be "pimping", only as being useful examples.