I think a 4x8 layout can work fine but the first thing that has to be set in stone is scenery or lots of operation. You can't have both in 4x8. The 4x8 posted by Harry G. is a great example of an operations oriented layout. Lots of switching opportunities with six industries, a passing track, and a nicely located runaround track. However, the buildings are going to be about the only scenery on this layout. There's no space left for even a small hill let alone anything else that might act as a view block. As Chip said, you'll see all the trains, all the time. Nothing wrong with that as long as operations are your main interest.
On the other end of the scale, we have Chip and Maxitrains. They both have layouts with relatively few operational opportunities but great scenery. Lots of cool scenes to create and look at but an operations guy might be bored pretty quick. I also like scenery so I would be happy with one that has more scenery and less track but I think we're in the minority from all the 4x8 layouts I've seen.
Finally, we have what Harold is doing. He's using a view block that goes right down the middle of his 4x8, creating two 2x8 layouts. One side looks like it's devoted to more scenery and the other side looks like it's going to be set up for more switching and operations. If you have unobstructed access to all sides of your 4x8, this might be a great way to combine two types of layouts into one.
So, I think the question is not just "What's the best 4x8 plan" but "What's the best 4x8 plan for my style of modeling". I don't think there's anything inherently good or bad about a 4x8 space but it can made good or bad if your don't think about things like this before your start.