What I didn't like about hand grenades was that since they could be fired at the same time as a weapon, they would create windows where a player could be killed instantly with combos. I felt being hit and being able to react was one of the key elements that made legions fun.
I think a good trade off would be that the player wouldn't be able to use a weapon when he/she is throwing a hand grenade. Combos would still be possible, but now there would actually be a trade off for getting that opportunity.
I was not able to play with them long enough, but I do hear that they made some radical movement possible. As long as it doesn't break the game, I would still like to have them. It might even be a beginning to a whole new set of secondary weapons. Flash-bangs here we come.
O, and keep in mind that adding depth to the game might not actually make it more fun for everyone. It just gives us who have gotten bored with everything else (skiing, weapons, CTF) something to do.
