Aside from the broken masks, another quality that differentiates some arrancar from normal hollows would be the position of their hollow holes.
While the hole is normally located at the center of their chest, both as a symbol of the loss of their heart and because that is where the (saketsu) chain of destiny is attached, arrancar often have displaced hollow holes for some reason.
To normal souls that location is where it's chain of destiny ties it to it's body, or where the chain is free floating.
To a hollow, that is where it's hollow hole is located.
It also serves a purpose for shinigami, as seen when when Ichigo was stabbed through his chest and abdoman (in an attempt to remove his shinigami powers):
and when Ishida shot someone for the same reason:
Since these two locations are apparently the cores of a shinigami soul, it follows that arrancar should have a similar construction, and from what Ishida did and said, it appears that they do:
This is second time that we see Ishida attempt to cripple someone in this manner, and clearly the difference is that this time he only aimed for the center of the chest. An obvious explanation for this (assuming that any arrow can pierce her flesh once she's incapacitated) would be that her hollow hole is at the center of her abdoman and additional shots are unnecessary.
This would suggest that arrancar sometimes have displaced holes to accommodate their shinihami powers, while it's too early to really conclude anything besides that, it may be possible to gauge how powerful an arrancar is by looking at where their hollow holes are located (I would guess that the best configuration would be at a location that is neither at the center of the chest, nor in the middle of the abdoman).
Any other ideas about this?