It's simply a very, very well-designed light, and since the state of high-power rechargable incans hasn't changed much since 1982, it's still pretty high-tech. It's probably the most physically durable light of its type, it's bright, has long throw, the charging cradle is the best by any maker, it can take a wide variety of bulb and lamp assembly upgrades, it uses standard batteries, it can be focused (if you value that), it's HA III, and it has a lifetime warranty. It's also not too expensive, since you can find them for $69.99 brand new.
Other than the company's legal practices, regular Mags are most often criticized for being outdated. In some cases this is true (the Solitaire and Mini-Mag should have been LED at least a couple years ago), in other cases it's just people wanting to hop on the anti-Mag bandwagon (the C/D cell PR-base flashlight market is still pretty big, and those style Mag-Lites are good for the price and target market.) Others complain about the beam quality, but Mag has acknowledged that it's lousy in order to preserve maximum throw. And of course the company's marketing is nearly pure BS.