(On phone, so will be very concise)

1. I think so. However, it remains to be seen if it is a significant or useful enough constraint to justify attention over others.
2. Maybe. Hard to know if it is that or a coordination constraint though. I think that the more significant issue is that many of our good ideas never encounter a critical mass of supporting resources to bring them to fruition. There are definitely many coincidence of wants related problems, where someone has a good idea that someone would do or fund but that person never hears of it.
3. A better pipeline for idea generation and implementation. Incentives to share, curate prioritise and implement ideas. I think that more open innovation competitions and idea incubators could be helpful.

Some thoughts (not to say ideas) regarding 3:

• come up with more ideas 
  • just brainstorming in a very unconstrained way on relevant questions (e.g. "babble")
  • trying some systematic ways to identify implicit assumptions in our existing beliefs and ideas, and questioning them
  • looking at existing entities (orgs, fields, causes, tools...) and thinking about how they could be different
• share ideas more effectively in the movement
  • encourage sharing in the first place (makes me sad to read of posts people started in the past but never finished)
  • good compression of ideas (e.g. short posts, descriptive titles, beginning with a summary)
  • make things easy to find via search
  • talk to other EAs about your ideas
  • get feedback early on
    • maybe twitter is good for this?
• actual implementation
  • a lot of ideas may exist, e.g. in the dusty archives of this forum, that nobody has ever acted on and people have more or less forgotten about or never heard of in the first place
  • some (or many) people may generally be more interested in thinking - maybe EA is implementation constrained rather than idea constrained after all? (but I guess there are a lot of constraints anyway, and they vary substantially by who you ask; so idea constraints most certainly are a thing, affecting some more than others)

Good question! Yes, an ideas constraint absolutely could make sense.

My current favorite way to capture that possibility would be to model funding opportunities like consumer products as I do here. Pouring more capital and labor into existing funding opportunities might just bring you to an upper bound of impact, whereas thinking of new funding opportunities would raise the upper bound.

This is also one of the extensions I'm hoping to add to this model before too long. If you or anyone else reading this would be interested in working on that, especially if you have an econ background, let me know!