In many FPS games, grenades are one of those things that annoy me due to the lack of ways they can be utilized. In most games you can just cook them, and in some, you can't even do that. Here's how I think you should be able to use grenades in TW.
Pressing the designated grenade button, G perhaps, should equip the grenade, not bring it out and immediately pull the pin on it! So by pressing G, your character takes out a grenade. To throw the grenade, you'd then press LMB. Of course, you should be able to hold LMB and cook the grenade. While cooking the grenade (so holding LMB), you should be able to scroll the mouse wheel up or down to throw overhand or underhand, with overhand giving more power, underhand less so. Scroll up to throw overhand, down to throw underhand. Releasing LMB throws the grenade.
One feature that I thought would be great for grenades is one that would allow you to toss them around corners. So, let's say there are enemies in a house, and you want to get a grenade in there. Normally, you'd have to stand in the doorway to be able to toss one in, meaning there's a good chance you'll be shot in the process. Here's what I'm thinking... when equipped with a grenade while standing next to a corner (in this case, next to an open doorway), throwing the grenade while facing the corner should result in you tossing it underhand around the corner. In this example, you're standing next to the doorway and equip your grenade. You face the doorway and press LMB. This results in you leaning your arm out and tossing the grenade into the room. Would be realistic and very useful.