I sometimes have problems understanding opinions on the forum. Not the difference, but where are they coming from. Senjutsu has an anti-verse between 4:15 and 5:15 (1 minute!) that's actual complete drop in dynamics and pace compared to any section preceding it. If you think that entire arena is going to sing this slow, anthemic-anemic part just 5 minutes into the show. This part is the slowest main part (not counting acoustic intros and alikes) Maiden does in its entire show. This part has slower pacing than Blood Brothers which is a mid set ballad.
But the biggest problem I have with it as opening/fierce track is its reliance to the meter. Simply kicking up a notch somewhere won't work as seamlessly as it works in Wicker Man, Aces High, TNOTB or even short non openers like Trooper.
It can open the show, but it will be more proggy type of opening, more of a visual effects/atmosphere type of a track than a straight out boom like Aces High, Wicker Man, or any of the other typical openers.
I don't want anybody thinking anything bad, but if you're kind of a fan that gets same type of energy and internal drive out of Aces High and Senjutsu, you're not appropriate for the metric. It's like having a "this is great" reaction to any meal served in front of you. Nothing against that, I certainly appreciate that you enjoy everything put in front of you, but most people don't.
Also keep in mind that average maidenfans.com concert goer will be in the fan sections or around fans. Elsewhere is different - I zoomed around one of the shows because I had several bunches of friends on different spots. Different parts of arena reacted differently to the TBOS material.
You will definitely see a "reaction" once first few minutes of Senjutsu pass and the novelty of show opening boom and that drum groove wears off on the general audience that doesn't know the song or the lyrics. Apart from the sporadic solo, there's nothing further in the track to hook them. If you think that 5 minutes in, half of arena is going to sing that slow part with hands in the air you're extremely fan-biased.