Goblins are an interesting army, purely because the actual units of goblins don't do the heavy lifting of the army. I have an Orcs & Goblins army, and in that instance, the goblins fill a very different role than in a pure goblin army. However, the pure goblin army is actually a bit better, I think, than a mixed army at the moment. Hopefully, GW will fix that shortly with the upcoming new edition, followed quickly by the O&G release.
You'll want to make sure you maximize the number of things like Spear Chukkas, Doom Divers, Fanatics, and Squig Hoppers you bring, because the individual goblin, no matter whether they're a common Goblin or Night Goblin, is -bad-. They win through static combat resolution, so you want to be sure that you've got enough of 'em in each unit. Popular unit sizes for the several Goblin players around here range from 35 to 40 gobbos in a unit with spears. Between the wacky shooting attacks and raw numbers at that point, they do fine. The reason you need 35-40 of 'em in a unit is because WS2 is bad; S3 is strictly average; and anyone who isn't another goblin or Skaven Slave is going to kill a bunch of 'em before they get to strike.
Of course, being one of the few people out there who finds misfires and general mishaps funny is a must for playing Goblins-- there's a lot that can go wrong when you bring three Fanatics per unit. However, those Fanatics are what'll make the army playable.
If you want to bring Trolls, you're going to need to bring either Skarsnik or pick up an Orc character (which isn't necessarily a bad thing-- a Black Orc Warboss leading a huge mob of goblins is actually pretty funny.) As trolls stand, Leadership 4 and Stupidity -really- don't mix, and Night Goblin leadership is only one better. Trolls add a surprising kick to the army though, and make it possible to (more) effectively fight elite infantry that makes it to your lines (Warriors of Chaos come to mind.)
Keep in mind you're not going to win any tournaments with Goblins-- it's a fun army, and they'll even win games-- but any army that Causes Fear is going to have them fleeing for the hills. Cavalry armies and armies that are just as numerous (read: Skaven) are also troublesome.