Why the distinction between "Disagree" and "Block"?

Block is one of the most important aspects of consensus decision making imo - it doesn't quite solve, but definitely goes along way toward mitigating the https://en.wikipedia.org/wiki/Tyranny_of_the_majority ensuring well-operating groups go out of their way to accomodate views of all inidividuals, not just of large "voting blocks".

On minor note - the "reason" textarea is currently optional, which I think is good for most positions, but it should be mandatory for a block.

@diogonunes some brief responses to your comment above, in the interest of fleshing out this discussion a bit more:

Voting choices should be objective and limited. That’s why referendums have Yes or No

I think there's a fundamental misconception here from the beginning that a proposal/decision is a "vote" similar to a referendum, with a clear "winner" and "loser". Traditional referenda are - in their nature - divisive and restrictive. What you seem to be perceiving as an advantage of referenda (clear, concise binary choices) many see as being their major flaw. In general, I think consensus decision-making methods are an attempt to redress these major flaws in traditional democracy, emphasising compromise, discussion and often the creation of and agreement on various "3rd-ways" instead of the restriction to simplisitic Yes/No options that can create conflict and competition between group factions.

10% Abstain, 40% Yes, 30% No, 20% Block - Yes wins.

This is a misunderstanding of what a block represents. For the purpose of explanation in the terms you're using, in this case "No" wins, as the blocks act as effective vetoes (to use traditional terminology).

However, in consensus, the idea of sides "winning" and "losing" should be dissuaded in general, and blocks will typically not play out as vetoes in reality - rather they are opportunities to revisit and renegotiate the proposal.

I can see your point, and you're probably right. I don't have the consensus mindset just yet, since I came from developing a similar app with only +1 and -1 votes. Give me time :P

I do think consensus mode should be optional (but default), for groups that use different decision making processes.
Right now Loomio doesn't really suit the processes of non-consesnus groups.

Disagree is as important as the block, used in different context. I can support certain things when i disagree on things like process. On principles the disagree may become a hard block. Both are great help in defining and bringing a consensus based group into deeper solidarity on the real issues.