Perhaps it's the superb hydraulic transmission, and the two independent engines that allow the loco to keep running even if one engine should fail. Or maybe, it's the wonderful roar! of the engines when pulling away.
Actually, it's all of those, and more besides!
It is little wonder that, back in the 'seventies when these locos were still running, I'd go all the way up to London Paddington, walk to the far end of the platform, just beyond the front of one of these magnificent beasts, and wait to see and hear its departure.
There's nothing quite like being that close to such an event (and it was always an event, however routine!) and I did it several times. One good bet was the 2230 Travelling Post Office (TPO), which was often Western-hauled; and the atmosphere at that time of night, after sundown, was something else again.
Oh, and yes, I too have been on a Western-hauled excursion, and hung out of the window just like those you see in the following video(!)
This short clip by traintelevision features one of the preserved locomotives in which I have shares, D1062 Western Courier (the other is D1013 Western Ranger, both pictured above), and is best watched full screen and played through a high quality audio system...