Quality and variety. When I was younger, I was pretty much restricted to drugstore brands because I did not have much money to spend. Going from drugstore eye shadows to MAC shadows and pigments was like night and day. MAC's eye shadows last longer, they are far more pigmented, and they come in a huge variety of textures/finishes and colours. Whereas drugstore brands only have a handful of colours or a handful of palettes with two or more shadows. I used to think of MAC as being expensive and something I'd only buy once in a while; now it's what I wear 3/4 of the time, and I feel the money is worth it.
I also think their pigments are the highest quality loose shadows I have used. I like mineralized loose shadows too, but I just feel MAC's loose pigments give me the best colour payoff, staying power and texture variety. They don't have the biggest selection (because a lot of pigments they release are LE or now DC) compared to some other brands (especially the online mineralized shadow companies that sell samples), but hey. You can't win 'em all.
In addition--and although I think they release TOO MANY--I do love the MAC LE collections. It always gives me something to look forward to, even though my wallet doesn't. Heh. Some of the collections end up being duds, sure, but most of them have some or a lot of what I end up buying. I love their themed collections too because it gives me a chance to stock up on specific products that I don't have (e.g. N collection for neurals, Metal-X for metallic colours, C-Shock for vibrant colours [not that I really liked C-Shock]). It's also cool that they come out with NEW products and NEW formulas because I end up loving a lot of these (e.g. Tendertones, Mattene lipsticks, Slimshine lipsticks).
Someone else mentioned B2M--this is another reason why I love MAC. They recycle, and you can get free stuff for being a dedicated customer. What other make-up brand can you say that about? None that I can think of or know of. The fact that they don't animal test is a huge bonus for me because I don't buy cosmetics from companies that do test.
Anyway, enough of that! MAC really stands out for me. There are a lot of other great make-up brands out there and I am not discounting them and saying "MAC rules all," but MAC definitely holds a special place in my heart and on my vanity.
Oh, and might I mention all of the wonderful people I've met being a MAC-o-holic?! That's another reason MAC is different. It has one of the largest--if not THEE largest--'cult' following. I love how MAC has brought millions of people together. Specktra is a prime example.