I'd say there are two key things to pay attention to:
1. Does the device have an officially unlockable bootloader?
Many Android devices do (e.g. Nexus devices, most HTC devices, most Sony Ericsson phones (though that may change as they are absorbed into Sony, the Asus Transformer Prime, ...), some do not (most Motorola phones, Sony's Android tablets and Google TV devices, etc.) and then there are some in the middle (they're relatively easy to unlock, but the unlock isn't official - e.g. most Samsung and LG devices).
2. Is the device getting an official version of ICS?
As I and others have noted elsewhere in this thread, ICS is a technically demanding upgrade (particularly at the kernel and driver level). If your device has gotten or is getting official ICS, then any custom ICS ROMs will have an official kernel (with source) and a full set of third-party drivers to build on, which is big leg up on devices that don't have that.
1. Does the device have an officially unlockable bootloader?
Many Android devices do (e.g. Nexus devices, most HTC devices, most Sony Ericsson phones (though that may change as they are absorbed into Sony, the Asus Transformer Prime, ...), some do not (most Motorola phones, Sony's Android tablets and Google TV devices, etc.) and then there are some in the middle (they're relatively easy to unlock, but the unlock isn't official - e.g. most Samsung and LG devices).
2. Is the device getting an official version of ICS?
As I and others have noted elsewhere in this thread, ICS is a technically demanding upgrade (particularly at the kernel and driver level). If your device has gotten or is getting official ICS, then any custom ICS ROMs will have an official kernel (with source) and a full set of third-party drivers to build on, which is big leg up on devices that don't have that.