Generalhenry,
aye, no more worry of the "death grapple by large beastie"
Monboesen,
a direct penalty to Defence would be wrong, but they do have limits on the max, ie you never get genius half giants (unless they put all their stat points itno Int/wis, which is fine by me).
bad reflex and will defences are a serious risk, which is a good penalty itself.
That's why I don't mind +4str/+4con and -2 dex/int/wis/cha, or some such.
In the 2nd Edition original boxed set rules, a halfgiant's max stats, after racial adjustments adjustments (and the fact base stats for everyone were up to 20, not 18!!), were:
S24 D15 C22 I13 W15 Cha15
If you lowered it to a base 18 for stats as maximum roll, max for stats as normal 2nd ed AD&D it would be
S22 D13 C20 I11 W13 Cha13
For racial bonuses in 2nd Ed they had S +4 C +2 In -2 W -2 Cha -2, but the racial max limits of course, clamped those down.
They also got double hit points *dice rolls* (not Con). Guess that was done by designers as 2nd ed was crap for working out high Con and large creatures?
In Dark Sun, since every PC has a WIld Talent (thus any bugger could have a fear, charm hold type power), and common enemies are Templars (who like mind affecting powers as dead slaves/criminals aren't as useful as live captives), belgoi (dominate and suck you dry, ick!), etc...it makes halfgiants good fun, but vulnerable.
You maybe an ass kicker but Joe Shmo could make you run away with telepathy, lol.
IMHO, for 4th, you don't really want any stat lowered below 15 max to start with, so you can play any class, if you wish, 15 *is* fine starting stat for any class.
I have no objection to a halfgiant playing a preserver (wizard)...rare oddities happen, why not! PCs = heroic oddities

Most halfgiants though are pretty thick, but friendly.
Hm, half the abilities sounds good....except it makes non-melee characters crap :/
But is that actually BAD? because a half giant clerics telepaths etc are rare and usually not so good...but the fact they can still kick your ass with a club does kind of maybe make up for it?
Hm, what about...
HALF GIANT
Large, Reach 2
+4 Str/Con, -2 Dex/int/Wis/Cha (or -2 on two scores, and -4 on two?)
Con modifier bonus to starting hitpoints. (meant to be very tough)
halfgiants get a racial at will, something like "Flattening Smash", using a melee attack, they do 1[W] damage + Strength modifier VS AC, Secondary vs Fort or be knocked prone for Medium or smaller creatures.
(that gives them a very logical power IMHO, and works in with losing 1 at will so they keep 2 at wills, can't use this as ONLY at will in case character uses ranged weapons etc?)
Halfgiants gain only 1 at will power, and lose highest level encounter and utilty power.
?
