Diminishing returns means 2 things, depending on context.
When it refers to stats like dodge, it means that adding the same amount of the stat (as in this chestpiece has 27 Dodge) adds less and less Dodge percentage the more you have. (For example, if you have 1% Dodge at whatever level you are, it might add another 1%, but if you already have 40%, it might add 1/4 of a percent. )
When referring to crowd control (spells that disable a player or mob for a significant period of time) it means that if you try to use that same spell or another that also disables the mob or player one after the other, the second spell cast only lasts for half as long, until after several casts it doesn't work at all.
Yes, JC makes gems. Except for some end-game enchants that provide gem sockets, they come on the gear (again, toward endgame). Gemming before endgame can get very confusing and expensive unless you can make your own, unless you make your own I generally don't recommend it.
Alchemy lets you make potions primarily, Engineering lets you make items that for the most part only you can use, and Archeology doesn't generally provide a lot of benefits although it does play a role in making a very cool mount (if you are lucky, and an alchemist) and has a few cool mounts/pets.
It sounds like you might want to browse through the various professions:
(Removed - Monmon's answer is more specific and precise than mine, go with his!)
Portals, you cast a spell once you are high enough level to have it, and then anyone in your party or raid can right click to use it.
(Someone else will have to answer the hunter question, and the warrior question)
Edit (Monmon already did while I was writing up a response!)
Edited by Sylth on 4/22/2013 4:13 AM PDT