Leaves in gutters isn't that big a problem, it's the removing of them that is the main problem to most folk as they can't get up there to do it. I often get asked to check someone's gutters as they are pouring over, but my ladder wont reach high enough for many, especially Victorian houses. Once you're up there a trowel and a bit of elbow grease is all you need. Moss also builds up causing problems but it's quite fun to pull it out as it tends to come out in one piece like a very long narrow piece of turf. One of the biggest gutter/downspout blockers are tennis balls. Children hit or throw them onto a roof, they land in the gutter then eventually follow the flow of the rainwater to the top of the downspout to block it completely
Bay windows also suffer leaks if water builds up on the flat leaded roof. Some i go to have about 4 inches of water on top. All it needs is to prod the drainage hole every now and then to keep it clear, but many don't bother so they develop a leak.