A Longwalker & all sort of other scouts can very easily be a hunter, not necessarily a Rogue. Longwalkers are basically messengers, not really a class but more of like a job? Just a fast scout. Hunters are very scout like with their tracking, eagle eye & agility.
I liked the majority of new race class combos that Cataclysm brought (Tauren Sunwalkers & Seers/Priests worshiping their sun isn't that crazy, especially because Tauren are very spiritual) but Tauren using Arcane (through non drudic means) or being dagger wielding assassins isn't really in-line with with the current lore & identity of the Thunder Bluff faction Tauren.
Same goes for Dreanei as a rogue, doesn't seem to fit in much with their morals or combat style. Especially Light-Forged. Too many class-race combos kind of ruin the identity of races.
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A scout/messenger can very easily be a hunter too, not necessarily a Rogue. Also a scout doesn't really have to fall into any one class really, it's more of like a job? Hunters are very scout like with their tracking, eagle eye & agility.
The orcs & elves, although tall & muscular still have a more agile frame then Tauren & are still smaller. If only we could adjust character body type a bit to look more lean it would be easier to envision. The Shattered Hand Clan are technically mainly Rogues.
Especially Night Elves (assuming you mean them by 8feet tall?) They can literally go invisible as a racial ability. They're very shadow oriented by nature (You know...Night & all). And if it is magical (technically subtlety rogues seem to be the only ones that use some form of shadowy magic) Tauren aren't big fans of it. Even lore wise, the majority of Tauren priests are light aligned & not really shadowy assassins. Maybe the Grimtome sure but as far as I know being dagger wielding assassins isn't really in-line with with the current lore & identity of the Thunder Bluff faction Tauren.
Same goes for Dreanei as a rogue, doesn't seem to fit in much with their morals or combat style. Especially Light-Forged.
Now I get Blizzard will probably write some Lore reasons to make them fit, but not everything is written well or makes sense just because it's there. OH WELL rant over
Too many class-race combos kind of ruin the identity of races. Tauren & Dreanei fit the role of the more peaceful spiritual races.