You have kind of heard my thoughts on this in a different thread, but i will try and give a but more detail here.
PvP participation in classic is VERY low compared to the original release of these expansions. This is due to a variety of things such as a stale pvp meta, no class/spec balancing, time investment required to gear and much more.
While i personally dont see rating requirements as a bad thing, part of this is bias as i play above the rating required for any items. For casual players, it has become VERY apparrent that the time investment required for casual pvpers to even finish a base set of pvp gear is well above what they are willing to commit to. Combine this with the fact that less players are living in ignorance and know just how handicapped they are going into BGs with full PvE gear and you see more players not even trying to get into pvp.
By removing all rating requirements for items it becomes much more accessible to non-pvp focussed players and encourages them to pvp to fill out some slots of gear in PvE due to the targetted nature of pvp gearing. This is completely fine, as PvP items are rarely ever competitive against their PvE counterparts in a raiding environment. High level PvPers are typically only interested in gear as to ensure an even playing field rather than to improve their performance.
Both expanding the pvp brackets and providing fun cosmetic rewards for the lower brackets (i.e. challenger, rival and duelist) could also go a long way to encouraging casual pvp participation. Examples could be unique weapon enchant effects, tabards, costumes, toys and much more.
More important to ensuring higher pvp participation than rating requirements is simply lowering the time investment. Using TBC as an example, it requires 10s if not 100s of hours to grund the honor needed to completely gear out a new character in gear 2 SEASONS out of date. This is a HUGE time sink only to still be at a significant gear disadvantage going into the arena. Blizzard has partly addressed this by reworking the arena point calculation to give significantly mlre points for lower rating, however, I would argue that addreasing honor gear is FAR more important than arena gear when it comes to boosting pvp participation.
The move to a personal rating system is a welcome change as it allows more flexibility when playing with different groups of people.
Lastly if we dont change anything and leave wotlk in its original state. I fear we will see a large drop off in casual participation as without a decent casual playerbase, reaching those rating requirements for gear becomes much, much harder. This fosters an environment where the only people participating in pvp ARE the min maxers, as many casual raiders will simply ignore even trying to arena as there are no rewards that interest a PvE focussed player AND reaching a rating to recieve any rewards is incredibly difficult.