1.Itemlevel. Once you pass certain itemlevel you will be put into the mid-gear bracket.
If you are close to the cap, you will be put into mid-gear bracket games if there are not enough players/off-peaks.
1.2 I am almost sure that gems and enchants does not have any impact on your gear or itemlevel towards brackets.
1.3 I do not know, but it does not make any sense to PvP in blues as even when you are full furious gear you are still in low bracket.
2. There is no official data and probably for a good reason, but after having characters in all brackets and playing for a while you will soon start to see patterns and figure out the caps of each bracket.
3.1 I would guess the ilvl bracket system uses softcap. Once you reach the softcap, you technically still play in "your" bracket but game will start to move you into higher bracket at low players and offpeaks. Thats what I noticed.
3.2 I have not notced this. All my chars switched brackets couse of geting better gear. I had one alt that I PvPed alot in low bracket with, and didnt outgrow it.
4. Hopefully
5.1 When the gear brackets were working, mostly games were fine (low and mid bracket). Sometimes blackrock players that rigged with itemlevel managed on purpose to get into low gs bracket.
5.2 Low bracket - Bracket is preety balanced but there are winstreaks, like one faction cant win for several hours until players on both factions go to sleep etc and rotate with new players. Usually those streaks happens in offpeak hours where amount of players in low bracket is not enough high for rotation.Can get stale if there is few hours of graveyard zerging going on. There are also the fresh 80s.
Green/Blue geared players 1k to 3k gs. It happens that the games are decided by amount of fresh 80s per faction. They do 0 damage and gets obliterated instantly by anyone with 200+ il (globaled). For example a good geared low bracket ret pala does one divine storm and kills 3 green geared 80 rogues.
Mid bracket is the Blackrock bracket. It's fun bracket with most players from Blackrock where they get instant 80 and prebis full relentless gear.
There are also Icecrown and Lorderon players here.
There are the "fresh 80s" there too, the new players that never played the class or even the game before and have no clue what to do.
Although they are not such a plague as the fresh80s in low bracket since they have full gear and can just mash some buttons to do damage.
You can just start a character here and have fun here with relatively balanced pvp (once gearbrackets are back).
If you for example play on Lorderon and gets transferred from low into mid bracket, be prepared to spend and farm for long time until your gear is on same level as the gear blackrock players gets for free.
+1 Low ilvl bracket, and you can get better gear than that and still play in low bracket.