Well, nades can kill in two ways. The direct blast radius is of course lethal to a nominal radius. Everyone knows this (one would hope), so upon seeing one land in their vicinity, an enemy will naturally run like hell in the opposite direction. Occasionally they won't escape the blast, but usually (if they see it incoming) they will. Hence, the grenade doesn't kill outright all too often when it's thrown in plain sight. As MoChief points out it's best used in those situations as a lethal flashbang--a distraction that is to be ignored with dire consequences. Especially if the enemy has only one way to run to escape the grenade, it can be very effective in drawing hostiles in the direction of your warm AK-101 barrel.
If on the other hand you're NOT seen throwing them, that's when the nade stands a better chance at killing. Beware that the enemy can hear you yell "Grenade!" "Frag out!" "Qumbula!" (sp?) when you chuck a nade though. Spawns, especially those in the process of being captured, are the #1 spot to nade spam for kills. The enemy's too distracted trying to cap the flag, and here's something that sounds stupid but actually is true: He's trying to neutralize the flag, and he sees a grenade land at his feet. One or two seconds of confusion ensue. He thinks, "Oh God, do I stay within the flag radius so I can finish capping this, or do I run away, let the enemy take it back, but survive the grenade? Oh, wait, that's gonna blow up and kill me, isn't it--"
And of course, grenades can prove lethal in choke points. Hallways/alleyways near spawns, gaps in fences/walls, ends of bridges...the narrower and more confined, the better. The terrain right around the Karkand hotel is a great example of a spot that always gets raped by hand grenades--and not all that surprising when you consider just how hard it is to avoid the blast radius of a frag in those twisty alleys.