While better, grenade throwing is still a lot to be desired, as instantly being able to take a grenade and throwing it from your belt while holding a weapon at the same time is fairly unrealistic in regards of the time it takes.

Maybe G will allow you to select the grenade while a second press of the G key [ as an example key] cooks it. Pressing down the second mouse key [as an example] and then releasing the key allows a grenade to be thrown while holding it down increases the distance it is thrown before release. However it shouldn't take too long to get the power of the throw to max [maybe 1.5 seconds].

Another problem is throwing the grenades around walls, doors and through narrow areas. Now Dslyecxi of Arma fame had a clip on how to maybe get around this here - http://www.youtube.com/watch?v=aGt8QbK61aE.

Also how to throw a grenade as an exmaple is here - http://www.youtube.com/watch?v=u44OImPmcYI &

The popular ACE2 mod used to have [maybe still does] various throw grenade modes here - http://www.youtube.com/watch?v=_8SElukJoYM.