That said, I also agree with Perko: 1[W] seems too much for an at-will power with the given effect, though at this time it seems about right to me for an Encounter power.
Well, there is a 3rd level encounter power called Sweeping Blow which
- marks all enemies nearby (no Wis limit)
- deals 1[W]+Str to each of them
- you get +Str/2 bonus to hit if you use proper weapon
We are talking about power which
- marks almost all enemies nearby
- deals 1[W] to ONE of them
- that's it
I would say it is balanced. Yes, it is probably very useful for very defender oriented fighter, but is it bad that fighter will be able to mark more than one enemy per round? It is not like he will mark all the enemies, because it will affect only adjacent ones - so I expect it to be 2 in most cases (monsters tend to avoid touching fighters regardless of being marked or not)
If you really think it is overpowered, then what about cleave-like solution? You deal 1[W] instead of 1[W]+Str, but mark one more enemy of your choice?