This is another oft-overlooked but important one for me, way ahead of fancy features like gradient fill meshes. Here's my two cents on what could be a really cool Affinity spin on the feature:
Imagine, instead of just selecting the same fill or stroke, you had a slider where you could "fuzzy select" similar objects. So you move the slider up a bit, and now you get those other objects where you used a slightly different shade of blue, or the ones that have a 1.85 point stroke instead of just the 1.5 point stroke on your selected object.
