In thinking about what it means to lead a good life, people often struggle with the question of how much is enough: how much does our morality demand of us? People have given a wide range of answers to this question, but effective altruism has historically used "giving 10%". Yes, it's better if you donate a larger fraction, switch to a job where you can earn more, or put your career to use directly, but if you're giving 10% to effective charity you're doing your share, you've met the bar to consider yourself an EA, and we're happy to have you on board.
I say "historically", because it feels like this is changing; I think EAs would generally still agree with my paragraph above, but while in 2014 it would have been uncontroversial now I think some would disagree and others would have to think for a while.
EA started out as a funding-constrained movement. Whether you looked at global poverty, existential risk, animal advocacy, or movement building, many excellent people were working as volunteers or well below what they could earn because there just wasn't the money to offer competitive pay. Every year GiveWell's total room for more funding was a multiple of their money moved. In this environment, the importance of donations was clear.
EA has been pretty successful in raising money, however, and the primary constraint has shifted from money to people. In 2015, 80k made a strong case for focusing on what people can do directly, not mediated by donations, and this case is even stronger today. Personally, I've found this pretty convincing, though in 2017 I decided to return to earning to give because it still seemed like the best fit for me.
What this means, however, is that we are now trying to build a different sort of movement than we were ten years ago. While people who've dedicated their careers toward the most critical things have made up the core of the movement all along, the ratio of impact has changed.
Imagine you have a group of people donating 10% to the typical mix of EA causes. You are given the option to convince one of them to start working on one of 80k's priority areas, but in doing so N others will get discouraged and stop donating. This is a bit of a false dilemma, since ideally these would not be in conflict, but let's stick with this for a bit because I think it is illustrative. In 2012 I would have put a pretty low number for N, perhaps ~3, partly because we were low on money, but also because we were starting a movement. In 2015 I would have put N at ~30: a factor of 6 because of the difference between 10% and the most that people in typical earning to give roles can generally donate (~60%) and a factor of 5 because of the considerations in Why you should focus more on talent gaps, not funding gaps. With the large recent increases in EA-influenced spending I'd roughly put N at ~300 , though I'd be interested in better estimates.
Unfortunately, a norm of "10% and you're doing your part" combines very poorly with the reality of 100% of someone's career having ~300x more impact than 10%. This makes EA feel much more demanding than it used to: instead of saying "look at the impact you can have by donating 10%", we're now generally saying "look at the impact you can have by building your entire career around work on an important problem."
(This has not applied evenly. People who were already planning to make EA central to their career are generally experiencing EA as less demanding: pay in EA organizations has gone up, there is less stress around fundraising, and there is less of a focus on frugality or other forms of personal sacrifice. In some cases these changes mean that if someone does decide to shift their career it is less of a sacrifice than it would've been, though that does depend on how the field you enter is funded.)
While not everyone is motivated by a sense that they should be doing their part (see: excited vs. obligatory altruism), I do think this is a major motivation for many people. Figuring out how to encourage people who would thrive in an EA-motivated career to go in that direction without discouraging and losing people for which that would be too large a sacrifice seems really important, and I don't see how to solve it.
Inspired by conversations with Alex Gordon-Brown, Denise Melchin, and others.
 I expect people working in EA movement building have an estimate (a) the value of a GWWC pledge and (b) the value of a similar person going into an 80k priority area, and this is essentially the ratio of these. I did a small amount of looking, however, and didn't see public estimates. I guessed ~$10k/y for (a) and ~$3M/y for (b), giving N=~300. Part of why I have (b) this high is that I think it's now difficult to turn additional money into good work on the most important areas. If you would give a much higher number for (b), my guess is that you are imagining someone much stronger than the typical person donating 10%.