What game are you guys talking about, IWD 1 or 2?
IWD1 EE. What are the differences between 1 and 2 that make thief worth more in 2?
Thiefs have more combat utility in IWD2. Also, i don't think there are any permanent traps in IWD2, though i may be remembering wrong. In IWD1, most traps can be triggered with a decoy, whether one of your tanks or summons, but some traps will trigger over and over and can only be removed with the thief skills, and some of their effects are pretty nasty.
The secret to IWD1 combat is simply not to get hit. HP is sort of useless, especially if you are playing on insane. This means that your best tanks are going to be summons and your magic users (this is one of the reasons i hate HoF mode. Summons get buffed too, so the game becomes this tedious grind of trying to take enemies down while summons are still up, rinse and repeat until the end). Dwarven Defenders can actually achieve immunity to physical damage which is broken as shit, but he is still vulnerable to magic. So you still need summons and all the protection magic can provide. For this reason it is a good thing to have your droods and clerics also be tanks, as as well as a figther/mage if you can squeeze one in your group (make an elf one too. The best shield in the game can only be used by elves or half-elves). Dualing from fighter helps, also because the EE follows the BG2 rule of being able to get grandmastery with even just two levels in fighter, which is total bullshit and game braking. The best combo for this is fighter 7 (where you get an extra half attack per round) and rest druid. Druids have a very fast progression at early levels which allows you to dual at such an high level as figther without spending half the game waiting to regain your fighter skills. Dualing from fighter will also allows you to use heavy armors and shields, and some of the later forms you get are very powerful. Especially the Water Elemental form, which starts with 75 in every physical resistance. You can easily get to 100 and with stuff like Iron Skins you have an exceedingly strong decoy. The AI is pretty straight forward and most enemies will blow their load on your decoys, summons or tanks, making the rest of the fight easier. Clerics also favor from dualing from warriors (again also because of the broken BG2 grandmaster system) but i wouldn't recommend pushing for level 7. The advantage of adding fighter levels to clerics is that you are allowed to go over plain 18 strength (you can start with Half-Orc to get around this if you want a plain cleric. More broken shit) and also get any constitution bonus above 16, which is useful when you are using Draw Upon Holy Might. For all this, dualing at level 3 is sufficient.
You now need DPS. For melee, dual wielding is king. Don't bother with two-handed weapons, Bioware saw to that. Make a Paladin, dual wield Pale Justice with that Long Sword of action, cast Draw Upon Holy Might, rip everything to shred. Best kit is Undead Hunter. Half the enemies in the game are undead. Cavalier is more useful against some of the stronger enemies, but the fact he can't use ranged weapons is too much of a penalty. You want your melees to double as ranger attackers.
Speaking of which. Archers are broken as fuck for this, but for IWD1 are actually overkill, because of all the bows that add so many attacks per round. The cap is 5, and you can get to that just with the skills the Archer has. For this reason, go for a Stalker instead. Backstabbing with fighter Thac0, lmao, retreat, switch to ranged.
What other broken shit is there, let me see. Ho yes, Sorcerer. IWD1 makes scrolls very rare and the game is balanced for that. Sorcerer bypasses this so you can have all the powerful spells you need well before the game would have normally allowed you to have them, ayy.
Bards are broken too. Warchant of the Sith is basically free healing and was broken even in the original. They fixed it in IWD2, after realizing their mistake. Skald is also quite powerful, not the least because his song also buffs summons (as do some spells, like the cleric Strength of One). Bards are best used for their singing (activate their script or you'll be driven insane since they stop singing as soon as you command them to attack), but the level 6 spell Mislead allows them to finally enter combat, since the decoy the spell makes can actually sing in place of the bard. It used to be that you could even stack the songs on top of another, but by some miracle this was actually fixed, i don't know why since all the other broken shit was left in. Lastly, Bards cast spells at their character level. This means that in any spell that benefits from caster level, they are utterly superior to mages, since bards level so much faster.