I have not read every codex. I haven't even read the entire rulebook in, oh, about 20 years. Maybe more.
But after checking out some Eldar and Eldar Renegades and Tyranids and the GK (I'm only there for the Coteaz + Henchmen stuff) lists I've come to a conclusion:
TOO MANY GRENADE TYPES.
How many does this game really need?
Rad - umm, ok. A little shoehorned Mary Sue stuff never hurts. Well, it does, but not that much.
Melta bombs - sure, kind of like a satchel charge thingy. All right.
Plasma - ok, well known 40K stuff that goes boom.
Photon - wait, what?
Empyrean Brain Mines - dunno if this is even a grenade.
Psychotroke - huh?
Psyk-out grenades - the first thing I ever looked at in a 40K rulebook and went "dude, that's a stupid name." Come on, why not call it something like "Anti-psyker grenades".
Haywire - ok, basic EMP thing.
Ork grenades - ok, sure, but why they couldn't just use the Krak stuff is beyond me.
Anti-plant - Agent Orange, sure.
Choke - what? CS?
Hallucinogen - do we need this and the Psychotroke?
Scare gas. Come on. How about just making that a possible result of the Hallucinogen ones?
Stasis - getting silly, now.
Tanglefoot - how about calling it "Web" or "Sticky" or "Glue" grenades?
Toxin - sure.
Vortex - still around?
Xeno filament - huh? Wouldn't that be peculiar to each race?
So what's the point?
The point is that there's too much stuff to keep track of for a new player - it should just be about 5-10 main choices and the rest "count as" or have a modifier to the result. There's only so much stuff you can do with explosives and their effect on an animal. You're an animal. Humans are animals. Tau and Eldar are animals.
Let's say that the Tau designer wants to give his footsoldiers a close-in anti-tank weapon. He designs a cool, fluffy little self-contained robot that runs around autonomously and kills tanks. Or "counts as" a double melta-bomb, rather. There's no need to give the effect a new name, like "Chumba-Wumba grenade Mk 3" - just say "Melta Bomb x2" and move on.
Same thing with the weapons. There's inflation in number of different weapons and their ammuniton. Each new codex just has to have some "new" stuff. But really - there's only so much stuff you can fire out of the barrel of a gun until you run into warp magic or whatever. Let's keep it to "Armour Piercing ammunition" and "Armour Piercing +2" or similar. Make things easy for a player to relate to, even if they haven't faced that threat before. I should be able to look at a list and immediately know if their weapons/ammunition are a huge anti-vehicle threat, etc.
EDIT: Fluff is nice. "Flesh hooks" for Tyranids is nice, and it'd be a bit silly to just give them Frag grenades. The "Flesh hooks counts as assault grenades" is a very nice way to give Lictors a small buff something while still keeping things nominally fluffy lore-wise. Solution would be to have the Army List info for Lictors read:
Lictor with Flesh hooks 6" 6ST Assault 2, Rending (also counts as Frag grenades)
That way any person not familiar with Lictors doesn't have to flip back to the Lictor-special fluff page to find the special rule, they just know that having the Flesh hooks gives the Lictor a pistol-type attack that also counts as having Frag grenades for purposes of Assaulting. Ta-daa! The fluffy-loving Tyranid player gets to keep his fluffy cool name "Flesh hooks" (because that *is* a pretty damn cool name) and the rest of us innocent victims get to quickly and easily figure out what the hell is up with this thing that's set to get sprung on my slice of cool mini dudes.