Pretty much the only real advantage will be normal BG honor; saved arena points reset but BG honor will not.

Looking at Burning Crusade, a new to 60 toon in mostly greens, with the odd 50th-60th blue piece, had absolutely no difficulty leveling in BC to 70th.
Sure, level 60 purples would be an edge, but greens allow for quick leveling, especially considering focus fire of multiple toons.

So whatever your gear level, if your toons are 70th for opening day of WotLK, you have an equal chance as anyone else in the march to 80th.
At 80th, everyone will have close to the same level of gear. Level 80 quest rewards will be superior to the current top arena/raid 70 gear.

The only advantage from BC to WotLK will be the ability to immediately purchase a few pieces of low-end (BG) pvp gear for your 80's.
That's assuming you PvP, if you're on a carebear server like me and exclusively raid/instance the field will be level in basically all regards.