* giant boulders - impassable terrain for PCs (or they can do an athletics check to climb it), however, the orcs can vault over the boulders. This, however, may or may not mean much depending on other terrain features around (i.e. for instances, the PCs might think they've cornered an orc, only to have him be able to escape. OR the PCs could think they have their back to the boulder for saftey and can't get surrounded only to have an orc vault up there and attack)
* the orcs should have a special power they can use when charing. Vault attack. +X vs. Reflex. Hit: X damage and push target one square and knock him prone (or something like that). Can cross through enemy squares as long as one square in the path is unoccupied (which is the square that the orc plants his pole, and is essentially leaping over the other squares). Special: This effect can be used in place of a charge.
* Loose small rocks to act as difficult terrain. A hinderance for both sides, though the orcs could vault over it a lot easier than the PCs to get better positioning
* Boulder roll. If the path the PCs take is the "obvious" / least-difficult path up the mountain, the orcs will undoubtably have that path with protections. So, at the top of said path is a boulder rigged to go. _If_ the orcs can get to it, they can release it (standard action) and the boulder rolls down anyone along a particular line (attack +X vs. Ref. Damage X and maybe dazed until end of next round). If you have a couple of these, the players will learn from the first one and will try and take down orcs before they can release the second/subsequent ones rather than focusing on the orcs immediately next to them.
* a mountain stream. have it somewhat shallow (so it can be walked across costing 3 squares to enter a square) and make it wide enough that no PC can fully cross in one round of double moves. Thus, as the PCs start to cross, ideally halfway across after a round, the orcs can come charging out and half can vault to the other side. Thus, the PCs are surrounded. and then ready actions to attack any pc that moves within reach range. thus, with those reach weapons, the orcs will be able to get an attack or two against the pcs before they can fully close for reach 1 weapons (or they have to change to ranged tactics -- and, if they do, the orcs can back to behind a boulder for protection)
(These are just off the top of my head... i know i'm kind of bad at this, but maybe one of these ideas will spark a better idea for you)