Double pin nades:

Fire1 = 5 second fuse, hold to 'cook'
Fire2 = contact fuse, as soon as the nade hits something it will go off.

Best of both worlds.

F.E.A.R.'s grenades had a fuse, but would "smart-detonate" if they hit enemies. It was good, because you could bounce them through a window or rolls them through a door, or you could go into super slo-mo and paste a guy between the eyes with a fastball of horror.

Since users are what in the end counts, here's what I observed in the few LAN parties I made, which are always good methods to observe "real" players. In my case, the players were ranging from newbies to average (some of them are beginners some occasional).

Grenades
What I have observed is that being killed because of a grenade slipped in the path is remarkably boring. If they have to wait before explosion, they should explode very soon. I would say that 1) they should explode after a few seconds on the ground, still being visible and 2) the player should not be able to have more than X (say 1 or 2) grenades around, meaning very slow throwing. I have seen a game (don't remember what) that allowed you to shoot a ton of grenades before the first explodes.

Grenades should be much more powerful that rockets - at least this is what the observed users expects - because they are much more difficult to use.
However, this hold true only for "fast" games. If the game is "slow" then grenades, being able to bounce, have a definite advantage. In this case, maybe they should be less powerful.

Reloading
Controversial at least. The observed pattern is that stockpiling players get less advantage because they can get only limited advantage (this is good) however, in all the games with reloading we played, everyone sooner or later screamed because he ran out of bullets. I would say it's game dependant but I'm confident reloading is good.

Shileds
Bad idea as far I've seen because they have limited visibility. I noticed players feel much better with a decent armor system... if you have one.