I don't think I could go round regretting things, it's the way you learn. There are a couple of obvious bad decisions of course. For example I had a '94 F650, a proper one built by Rotax. Decided I needed more power and bought an R1100R that was a pointless badly made piece of junk. I was then offered about the last R80GS ever made, but like the idiot I was back then, believing the adverts, I thought I'd have another new F650 instead. That had been got at by the Tractor Company's cost cutters and promptly died in the desert due to the famous load bearing lip seal in the water pump. While I regret not getting the R80, it would honestly have been a waste, I'd have got bored with the oil changes or fiddled with something and wrecked it and sold it on, so I can't really say I regret it that much. Now if I was offered that bike in that condition now.....
I think people need to go through the stage of not knowing what they really want, or is that heading towards Zen or something. For me, I've settled on Bonnevilles and MZ's. Only addition I really fancy is either a Rotax-zed or a Scrambler.
Andy