Other than what's been said about downloading/Amazon/eBay &c, surely an overriding factor is lack of spending money from a lot of people that has taken a toll on HMV? Even as someone with a music blog/fanzine, who would class themselves as a huge music fan for a significant sum of my life, I will freely admit that I haven't been buying as much music as I once did. Albums are quite a luxury to me now, as much as it would pain me to say and think about. My enjoyment in them has not abated, but hard times and responsibilites and adult life and all that.
When I was in my teens and early 20s, I could spend in music shops frivilously without much thought. I would adjust my pocket money then in later years my student budget/bedsit budget to whatever albums/gigs I desired. Eating came second, and I would prioritise my weekly food allowance against gigs and albums. Memories of living on gone-off Polish bread, cold beans out of a tin, a chocolate bar as dinner, and so on - as long as I could get out to my gigs, or get that new single or album.
Living in a provincial town before I went to university, I had - at the peak - four mainstream music shops to choose from (HMV, Our Price, Andy's, and I forget the fourth but can picture myself queuing up to buy singles on a Saturday), then it dwindled to two (Virgin 'Mega' Store, and HMV), then just HMV. But moving to a city, I could shop at the big HMV on Oxford Street (quite a regular Saturday joint or meeting/browsing point, a perennial, known fixture), but then I had all this amazing selection of independent record shops on Berwick Street, and a plethora of second-hand joints to bargain hunt in. If my teenage years were filled with Mondays and Saturdays using up all my pocket money to buy up a handful of singles, then my 20s was where my album-buying really stacked up - Music and Video Exchange serviced my collection with an archive of every 90s indie album I could want, pretty much.
In later years, HMV was always around, and believe me I did and do shop there (Christmas and birthday presents for friends were bought there most recently) - and also believe me, for years I have been concerned about its longevity and would shop there when I could (in fact, when someone recently asked me which 'download' store I'd like a gift voucher for, I requested instead HMV) - but as I got older, it became a concern that independent record shops needed my custom/support significantly. I shop in places run by individuals/independent businesses more often for this reason (read the book or watch the film of The Last Shop Standing, if you care about music and haven't yet, please). I think the problem also reaching my late 20s and my 30s is that unless I visit huge branches of HMV, the selection of stock is often like being back in the playground - got, got, got. The back catalogue of stock seemed to get lost and there'd be multiple copies of the same latest album on racks at times, too. Conversely, though, I still love the day when I recently went into my hometown's HMV (a medium-sized branch on two floors, but not a town with a noticeable indie crowd, at all), and saw obscure albums by Beat Happening and other lesser known indie bands - pleasingly obtuse. There was also the time I found a copy of the Mark E Smith spoken-word solo album (muttered stuff about orange tents and tomatoes, god Knows!), and when I reported back to someone they said it was rare, even deleted. I've got vivid memories of buying really great, life-changing stuff (and, even, old reciepts tucked inside old albums of mine).
My greatest sadness right now about HMV is that the one closest to me also has a cinema which it owns. A cinema which regularly shows independent, art-house films and that for a small pocket of London, is really quite something. The bar is also the best place to meet round here if you are interested in your culture/alternative leaning stuff. There might be what some consider a 'faceless' big name brand store on the ground floor here, but on the top floor is a cinema run by staff who put independent thought and effort into the music they select to play (over the years, have heard Yann Tiersen, Thom Yorke, War Against Drugs, Jeff Buckley, Robyn Hitchcock, British Sea Power, and stuff I couldn't name), and they are interested in talking about such stuff with you. This place has been a core place for me to place issues of the paper version of this very blog you are reading - which the staff have positively commented on, and which has proved welcome to people taking an issue home with them.
So it's that that gets me the most about HMV's situation in this moment. To lose the only decent indie place to socialise in locally for me, which also shows great films, and which also houses the only music shop round here, would be a huge personal blow to me and those I know who have visited with me.