Biggest deal for me is probably range of stock. It seems to be more and more common that I go to B&Q or Wickes and come home empty handed or at least not with everything I went to get. That might be them getting worse or me getting more adventurous or more likely both.
Don't waste space on scatter cushions, carpets and fitted kitchen displays like B&Q is currently doing. I want to buy ironmongery, plumbing fittings, electrical parts, timber, building materials and so on. When I want furnishings (and it's not that long since I had a whole empty house to furnish) I don't go to a "DIY" shop.
Have a decent website which displays all your products. There's no need for an online sales facility (for me, anyway), I just want to see what you have (and at what prices) before I go, so that I can plan the project. Decent pictures, dimensions, etc, not just a one-liner cribbed from the supplier's catalogue. Live stock figures for my local branch (it's 2010, don't tell me your stock-control system can't talk to your Web servers) so I know I won't have a wasted journey. A search facility that doesn't suck (get a Google appliance in if you can't do it yourself).
A few kiosks around the shop where I can (probably on a lightly-modified version of that same decent website with good search) search the catalogue to find out whether an item is stocked and where it is (ie shelf number). I seem to spend a fair bit of time wandering around looking for things, before I can find a member of staff to tell me where it is (or that they don't stock it; see above).
Reasonable opening hours. B&Q's are ok; if you close at six you won't get much custom from me, since I tend to drop in on the way home from work during the week, acquiring the materials to do the job at the weekend. Decent opening hours all weekend would be good, so lobby the government to stop restricting everyone's activities based on some people's religion :-)
I don't really need advice from staff - I get that here :-) True experts could be handy I guess, but that's hard to achieve and any less is not really useful. I don't go to a shop to find out how to do something anyway, I go to buy the materials (and occasionally tools) that I need.
It would be nice to be able to buy more in the way of timber, eg a nice length of beech or oak as well as carcassing softwood and MDF, but to be fair that might be a bit specialised for a general "DIY" store.
I do occasionally find automatic checkouts handy as an "express lane" when I've just nipped in for a packet of screws or whatever, but for any more than that they're a waste of time and highly annoying. Keep the staffed checkouts and limit the auto kiosks to perhaps two, with nowhere to put piles of stuff plus suitable signage, so that they just get used by people with a couple of small items in hand.
...and just to cap it all off it would be good to match TLC et al in prices. If you did everything else I've suggested but weren't the cheapest I would still use it for "everything in one place, right now" convenience, but you'd miss out on my big orders like bathroom refits, workshop wiring, etc.
Pete