How to decide whether the amount of study material provided to students is enough? Arguably the most time-consuming component of teaching is preparation of study material (especially when in comes to a fresh lecturer of a new module). 

First of all, every presentation should have slides. Who takes a lecturer without slides seriously? (with the only exception of, perhaps, really senior professors). If one dares to use a black- or whiteboard, students will take pictures on their iPhones instead of following the lecture, pretending they will follow it at home, which they never do. And students who miss the lecture will complain, since the slides are not available.Secondly, some question / exercise sheets are important to keep students entertained and busy in class / tutorial. Question sheets imply answer sheets, with (some) of solutions worked out, to help students prepare to the assessments. Lecture notes, to collect all relevant information in one place, and save students some trouble looking for it in various textbooks. Helps with those students who are hip enough to never use a library. Practice instructions, to carry out lab classes.

This is just a basic list, which comes to my mind, and I'm sure that other types of study / supporting materials are used in teaching.

I've heard rumours about some loci amoeni universities in the US, which provide their lecturers with all (or most) of these materials, professionally prepared by academic publishers. Apparently, this is not the case in the UK, at least not at every university. And I can now appreciate, that preparation of all these materials to every lecture / tutorial class can take a very considerable time.

Question: How do we decide, which materials are the most important for the class, and which are not? How do we assess, that the amount of materials prepared is sufficient?

Specs (by popular request): I am teaching Maths at BSc and MSc level in the UK. However, I would appreciate answers regardless your discipline / profile.
Use backward design, start with what you'd like the students to become or to achieve at the end of the course. Then, according to each of these "wishes," determine the best way to collect evidence on learning, and according to each of these evidence, design the teaching/learning experience. Formats should not drive the contents; it's the experience you wish to achieve that drives the formats. If lecture with slides is the best way, then lecture with slides; do not add lecture with slides because it's your institute's status quo.

Here are some of my anecdotes addressing your points:


  First of all, every presentation should have slides. Who takes a
  lecturer without slides seriously? (with the only exception of,
  perhaps, really senior professors).


Teaching in a US university, I have started to see students showing excitement because a certain lecturer does not use slides in the class as early as 2010. That got me start thinking if slides are still a viable teaching tool. After some thought, I made the decision to go slide-free and have complete done away with slides two years ago.

I like the format so far because slides had been acting as an invisible wall between me and the students. Just because I have clicked through them does not mean I have taught them. However, I felt safe behind the screen, and perhaps had grown to relish that false security too much.

I replaced the slides with what I call a "learner's package" (more details below) and I immediately felt a lot better. There are more discussions, questions, and interaction. The questions became a lot more spontaneous. In the beginning it can be daunting, but once I got into the rhythm I can never go back to the slides. As of now, four cycles into this new scheme, 80 students taught, and none of them complained the lack of slides in the evaluation.


  If one dares to use a black- or whiteboard, students will take
  pictures on their iPhones instead of following the lecture, pretending
  they will follow it at home, which they never do.


It's not if they will or will not follow up; it's if you have structured the activities or evaluation exercises to make them follow up or not. If there is no reason, why would they be motivated to look at something they have already archived?


  And students who miss the lecture will complain, since the slides are
  not available.


Many technological solutions here: e.g. First, make it clear in the syllabus that it's their responsibility to solicit notes from their peer if they miss a class. Second, consider video-taping your lecture. Third, consider using screen capture with audio recording if you will be using slides. Talk to your IT folks for possible supports.


  Secondly, some question / exercise sheets are important to keep
  students entertained and busy in class / tutorial.


While I love the idea of enriching the types of formats and interactions, it should be based on the desirable outcomes of the course, and not based on entertainment (though I sense that your questions are portrayed with a bit of sarcasm). Different types of format have their pedagogical pros and cons and they should be used to best fit what you want to achieve.


  Question sheets imply answer sheets, with (some) of solutions worked
  out, to help students prepare to the assessments.


I'd disagree with making it a routine to provide answers; this can cause them to rely on the eventually available right answers. Instead, I'd suggest base the questions on a defined set of skills (that you can point to at a section of the text, or on a certain page of the notes) with mild to moderate modification. That way, you can point them to the section to see how similar questions are solved, and encourage them to revise their answer.

If possible, base your answer scheme on instant feedback of the class's performance. E.g. if last week's quiz shows that only 35% of the class got question 6 correct, then consider making a detailed answer scheme for that.

Use your teaching assistants. Make them try your questions. They can i) find typos or errors, ii) give you a sense of how much time was needed to complete each question (multiply it with a factor of 1.5 to 3.0 to factor in the fact that the TAs probably can complete the exercise in a shorter time,) iii) draft the model answers.


  Lecture notes, to collect all relevant information in one place, and
  save students some trouble looking for it in various textbooks. Helps
  with those students who are hip enough to never use a library.


I agree a one-stop reference point would be helpful. It can be a set of notes or a list of materials hosted on the online learning website (like Blackboard.)

In my "learner's package," I included the followings:


List of learning objectives
Learning resources, which include corresponding sections in the textbook, readings, videos, etc.
Self-assessments. This section contains a list of activities and exercise with answers for students to test their own understanding. It's not graded.
Lesson evaluation. This section details how the students will be evaluated formally. I often put the link to the weekly quiz here. And if appropriate I also suggest some mile stone that they should achieve in this week with their personal final project. (I provide rubrics and so, but this answer is long enough so I'll save it for another day.)
All the notes, with citations.
In-class exercise and activities.
I use the Tufte LaTeX codes to compose the notes. I picked it because of its pleasant layout and ample amount of space for students to write.




Lastly, here are some titles that have inspired me with trying different instructional designs (I'm not affiliated with these authors nor Amazon.com):


Wiggins and McTighe. Understanding by Design.
Smith. Conquering the Content. It's for online course design by the contents helped me greatly in designing the online teaching platform.
Dirksen. Design for How People Learn
Ambrose et. al. How Learning Works
Davis. Tools for Teaching