So I'm awake. My biggest question is about the redundancy. Why would I use a Storm Rifle versus the Suppressor versus the Assault Rifle? One thing that Reach did really well was to remove the redundant weapons that cluttered things up. Admittedly, they added in the Repeater, which was essentially a shitty version of the AR, but that was so the Elites weren't hyper-powerful in Invasion. But overall, there wasn't much overlap.
When you say "pretty damn refreshing to see that power weapon list so minimal" I see essentially 19 power or semi-power weapons above it.
The Loadout weapons are deliberately repetitive; since everybody can spawn with them and they'd all hypothetically be unlocked from the start. It's mostly a matter of personal preference per individual category. To offer a counterpoint, while you may not see a reason to use the Storm Rifle over a Suppressor, someone else might, given that it has the highest ammo count of the three primary automatics. Let me try and provide a comprehensive list of mechanics.
BR: 4-shot kill.
Carbine: 6-shot kill.
Light Rifle: 5 or 4-shot kill.
All kill times are roughly the same save for the scoped Light Rifle which is the highest, but this is offset by it not being hitscan and being so "lightweight" (hence heavy flinching). Range reduced to BR and Carbine levels, Halo 4 Light Rifle range is inherited by the Pulse Rifle (see below).
Assault Rifle: Best at low-mid range.
Suppressor: Best at close-range.
Storm Rifle: Neither, but has the highest ammo capacity.
These weapons are roughly unchanged saved for possibly giving the Suppressor an altmode.
Magnum: Increased dexterity on things like reloads.
Plasma Pistol: EMP and homing functionality. The EMP isn't as powerful anymore on vehicles but the individual shots have been buffed, similar to the Reach/Reach Beta Plasma Pistol.
Boltshot: Charge shot eats up your entire clip and can only be used when you have a full clip; it's only strong enough to deplete someone's shields. Still a 10-shot kill otherwise. So essentially you're choosing between a beefy Plasma Pistol that can EMP or a quick Plasma Pistol that can headshot.
Silenced SMG: Pretty similar to the ODST version; major advantage is the scope.
Plasma Rifle: Slightly nerfed. It has a slower rate of fire but the "globs" of plasma are bigger and induce slowdown like the CE version.
Oscillator: Highest learning curve of the loadout weapons. When mastered it has a kill time of about the Assault Rifle but requires much less ammo. It has an altmode which is essential to successful use. Its primary firing mode shoots small red needles like Enforcers do, good against shielding. Tapping the scope button activates its altmode which fires a bright red beam that devastates unshielded targets. Using either mode by itself will result in below-average kill times; timing the enemy's shields dropping, switching modes, and beaming them is a matter of finesse.
Frag Grenade: See Halo 4 version.
Plasma Grenade: More lightweight than the Halo 4 version, detonation time after a stick is between the CE and 2 versions. Damage is halved against vehicles. One idea to prevent trolling is a "Bouncing Betty" custom game option that makes grenades thrown onto teammates' vehicles simply bounce off with no negative consequences to the vehicle.
Pulse Grenade: Made slightly heavier. Default mode saps your enemy's AA meter and has damage equivalent to what it is now. Altmode (hold the left trigger after throwing until the grenade goes off, no cooking) has no AA effect but induces heavy vertical and horizontal slowdown; damage is halved compared to AA Sapper. If your opponent either has no AA or it's recharging/depleted, the grenade's damage is doubled.
Phase Grenade: Default mode is a Promethean Firebomb. Altmode (hold left trigger until detonation, no cooking) is a Promethean Spike Grenade.
You can spawn with 2 grenades, using any mixture of the above 4.
Shotgun: See Halo 4 version.
Combiner: Needler Shotgun. Highest range of the shotgun but only 3 shots per clip, moderate rate of fire (think slightly faster than a Binary Rifle). The Combiner shoots out a huge flurry of needle shrapnel which homes in, but all the shards have to hit to induce detonation. Homing is on par with the CE Needler and takes just about as long to detonate.
Scattershot: More consistent spread. Charging the gun will shoot out the same projectiles but they have two and a half times the range and can ricochet up to five times before dissipating. This takes up two rounds in the clip.
SAW: Pretty much unchanged.
Needler: Halo 4 version.
Sentinel Beam: Basically a combination between the Halo 2 and 3 versions. Kill times are between the SAW and Needler but may have an altmode to make up for it (such as a hitscan beam that eats up twice the battery and overheats twice as fast).
DMR: 6-shot kill. Designed explicitly to counter Snipers and ping people from high ranges; in a fight against any precision loadout weapon where the Loadout user has a red reticle, the DMR user will likely lose.
Needle Rifle: Slowest kill time of the Precision Marksman weapons, but compensates for it with superdetonation functionality. See above for sandbox role.
Pulse Rifle: Halo CE Pistol and the Beam Rifle had a baby. High learning curve. Potential for a 4-shot kill with the third-highest kill time in the game for non-instakill weapons, the top two being the Sniper and Beam Rifle, respectively. It runs on a battery, so improper timing will make you overheat before you can get the kill. Not getting a kill on the fourth shot will cause a very loud, very lens-flared overheating of the gun. A kill on the overheating shot dampens the light and sound significantly as a reward for pacing your shots. This gun has horrible battery life, but its altmode (scoped) is a Power Saver mode that has a much slower 5-shot kill (equal to DMR kill time) that eats up considerably less battery per shot. It has a red 6-dotted laser sight visible on targets at all times. Range is about halfway between the BR/Carbine/Light Rifle and DMR/Nerfle; that way you can counter snipers, but aren't 4-shot pinging people across the map and causing another Halo 4 DMR situation.
I'll get into the other weapons later. tl;dr Loadouts are meant to be diverse so that you can customize your starting weapons according to your playstyle while still offering a balanced experience. Initial Ordnance is diverse because they're the only power weapons that spawn at the beginning of the match, and having 6 DMRs, 4 SAWs, and 2 Shotguns on each map at the start would admittedly be boring for a lot of players.
