Assault is the main CQB/explosives class and the gadgets are as you'd expect: TNT, Geballte Ladungs, a mediocre rocket gun and mines. The MP18 is pretty good for a starting gun, the M1918 has great hipfire if you like that type of gameplay and the Trench is a straight upgrade to it, while the Storm has better recoil. I'm ambivalent on the shotguns and have just been using the M97 Hunter.
Medic, pressing Q while aiming at somebody automatically throws the small health pouches, same with support's ammo packs. The Cei-Regotti feels lackluster, the M1907 is excellent at close range, the Mondragon and Selb are better at range. I tend to set them to 2x magnification instead of the usual 1.25 for irons.
Support really needs another type of gadget. Mortars aren't hugely useful on some maps, same for limpet mines. The starting Lewis is serviceable like BF4's Ultimax, the Hotchkiss M1909 Storm is a great "assault rifle" type LMG for before you get the BAR, the MG15 nA LW is a more conventional belt feld but is worse than the Lewis LW IMO.
Scout's flare gun is the most useful gadget and is essentially the recon balls from BC2, fire it into a area you want to spot stuff. It also burns people to death. I don't like the SMLE, but the M1895 Russian is a great rifle. K-bullets are kinda pointless unless you've got a support feeding you ammo, but I've downed a few aircraft with them.
Sidearms, just use whatever is cool. The assault's Gasser takes forever to reload but is fun. Apparently the higher rank melee unlocks can be used to smash through walls or damage vehicles. Mini and incendiary grenades are worth unlocking. Also bayonets increase your ADS time so you might find it worth removing them from some guns.
Don't spawn in the fucking heavy tank. Both the medium and light are far more useful. Vehicles have loadouts but they're fairly obscured. Haven't tried much with airplanes yet since I'm still working on setting up a good control scheme.