What kind of effect does the style of your gloves (e.g. melee, ranged, magic) have? I heard only helm, plate, legs and shield slot have effect on your offensive capabilities. If it is so, what is the point of using different class gloves/boots?

Only body, legs and shield influence your accuracy.

 

Different boots/gloves have different defensive stat spreads, so glaiven boots will have more defence versus magic whilst ragefire boots will have more defence versus melee, even though they have the same armour bonus.

 

Additionally, the defensive stat spread of your gloves and boots will be factored in along with the rest of your set, allowing you to skew your defensive bonuses in a direction you'd rather go. For instance, if you equip a pair of Steadfast boots (194 armour, good versus ranged) while wearing a set of Virtus robes (poor ranged defence), your ranged defence in your equipment screen will go up by much more than 194.

 

You should give it a try, it's easier to play around with them and the equipment screen to see what I'm talking about :P

It's nearly always worth wearing fully of the correct class, seeing as mixing penalises & wrong classed items don't exactly help you that much in most cases. I mean sure you could strap on mage boots rather than melee as you have a better pair with higher def rating, but on mage boots most of that def rating goes to melee, some to mage and very little to ranged where as melee boots give most to ranged, some to melee and little to mage; and in most cases a melee-weak monster would be inflicting ranged damage.

 

Edit:

I should probably clarify when saying nearly always I mean in combat as a whole, I'm not a big pvmer but I imagine the majority of the boss monsters are diverse enough to warrant more of the mixing, where as slayer is dominated by sticking to the triangle in all bar like 2 or 3 cases.

Using wrong style of boots\gloves does not give any kind of penalty, body,legs etc does.