Is there a way to simply prioritize traffic so an IP address or subnet has priority to bandwidth on a WAN link?

Depends on your definition of simple.
http://wiki.mikrotik.com/wiki/Manual:Queue

priority (1..8) : Prioritize one child queue over other child queue. Does not work on parent queues (if queue has at least one child). One is the highest, eight is the lowest priority. Child queue with higher priority will have chance to reach its limit-at before child with lower priority and after that child queue with higher priority will have chance to reach its max-limit before child with lower priority. Priority have nothing to do with bursts.

So: make a queue tree that is attached to the WAN interface. Make two child queues in that parent queue. Mark packets in the traffic flows that are to be prioritized with a specific packet mark, mark all other traffic with another mark. Assign priority 1 to the first child queue, and fire it on the marks attached to the traffic that is to be prioritized. Set reasonable limit-at and max-limit values. Assign priority 8 and the other packet mark to the second child queue, and again set limit-at and max-limit values.

Alternative make a simple queue attached to the WAN interface, and add two child simple queues to it. Set the priority to 1 on the first, and specify the target address you want to prioritize (host or network). Set limit-at and max-limit to reasonable values. Assign priority 8 to the other child simple queue, and again set limit-at and max-limit values.

The queue tree approach is potentially more powerful as you can mark packets on arbitrary criteria.

There is no "strict priority" queue where traffic marked specifically is always transmitted first.

Also keep in mind that QoS is pointless for prioritization for return traffic (packets coming from the WAN to you) as they've already passed the choke point - once packets are at your interface and you can do anything with them at all it's too late to do anything sensible with them. It can make sense through the rest of your network, though, if you need to prioritize along the entire path that you have control over.