Posting the "before and after" pictures of my tank got me thinking about the importance of a few key elements. The biggests problem with the "before" shots isn't the lack of plant cover, it's the fairly uniform and boring layout of the sediment. If I were to do it again, I probably wouldn't go for black and white gravel, but I would definitely start things up with a lot more rocks and wood.
If you have one big open space, fish use the tank as one big open space. Adding plants in the back and sides can create smaller refuges, hiding spots, but that doesn't offer any additional options with respect to swimming. The fish can hide or the fish can swim. But that's about all. When I added the driftwood in the centre of the tank I created an up-front hiding space (without it I would probably never see the kuhli loaches), but it still doesn't change swimming options. A shy fish can dash from cover at the edge to cover at the centre, but it's still a dash. But scattering rocks across the bottom of the tank (and adding plants between the rocks) changed the usable space dramatically. Suddenly the bottom of the tank was usable to relatively shy fish. It certainly made some of the corys more active during the day. This fairly small change led to created a new microhabitat encompassing most of the bottom of the tank. It should have been obvious, but it wasn't.