In reply to keith-ratcliffe:
I have seen almost every band I've ever wanted to see (helped by the fact that I don't really like seeing bands in huge venues so it's easy for me to "not want to" see Leonard Cohen etc).
In terms of stuff that I could and should have gone to, there are only two specific gigs that I regret skipping.
Life Without Buildings, some time in 1999 or 2000 (me being tight with money)
HangedUp, in 2004 (me forgetting to go to the gig as I'd just got back from a festival)
I can probably still get to see some form of HangedUp or other music from the members of that band.
For a long time I regretted never having seen The Triffids although given that I was 14 when they split up and never heard of them until I was 18, this was fair enough. Fell in love with their music, then Dave McComb died and I thought "well I'll NEVER get to see them NOW" but then they did do some gigs, tastefully with a roster of vocalists tackling different songs, and I saw them at The Barbican in 2010 and it was wonderful. Saw them again last year with a rather trimmed down roster of "guest stars" (mostly Rob Snarski, a lifetime associate of the band), still very good.
Aussie jangly indie band Even As We Speak are one that with a lot of effort I might have been able to see when I was 17. Happily they are now gigging again so I'll keep an eye out to see if they come to the UK.
Otherwise no real regrets
Post edited at 10:04