For an Avenger, Mordenkrad would actually be my last choice in superior weapons. There's no stat synergy for high Str/Con, and it's missing the crucial High Crit property. It'd cost way too many vital stat points to qualify for Hammer Rhythym. Hammers are great for Defenders, not for Avengers.
Hammer Rythm simply isn't as awesome for avengers as it is for everybody else. The loss of the High Crit property does hurt- I will certainly grant that. But- at least until you get out of the heroic tier- it's only slightly better on 1[w] attacks due to high crit and it falls behind on 2[w] and 3[w] attacks. The Mordenkrad also has the fairly subtle advantage of a much, much lower standard deviation when you actually hit something:
1st level avenger with 18 wis, Mordenkrad, swinging against a 3rd level skirmisher/lurker: Hits on an 11+ for 12 damage average (1[w]).
1[w]:
39/400: crit for 16
261/400: hit for 12
100/400: miss
Weighted Average Damage: 1.56 + 7.83 = 9.39 DPR
Weighted Average Hit Damage: 12.52, Hit sigma = 2.30
1st level avenger with 18 wis, Executioner, swinging against a 3rd level skirmisher/lurker: Hits on an 11+ for 11.5 damage average.
1[w]:
39/400: crit for 21.5
261/400: hit for 11.5
100/400: miss
Weighted Average Damage: 2.10 + 7.50 = 9.60 DPR
Weighted Average Hit Damage: 12.8, Hit Sigma = 4.42
The average hit damage is the amount that you connect for when you actually connect- it's not taking into account all the times you whiff for 0 damage. I calculated it to show the big difference in standard deviation. A pretty large chunk of the axe's damage is tied up in critical hits- fortunate rolls that you wont get more than a couple times a session, if that often.
Though the axe connects for slightly higher average damage (2.2% higher, to be precise), it also connects for sub-par damage significantly more often- it hits for less than 10 damage 26.1% of the time, while the hammer hits for less than 10 only 10.4% of the time. I judge the hammer's consistency, along with its damage advantage on hits with 2+[w], to be superior to the axe's meager average damage advantage on 1[w] hits. This will change in paragon tier, with epic tier bringing the two into slightly better parity.
What weapon feats would you judge to make the executioner's axe better later on?