@Doesn't understand:
Not much, except for the fact that the competition is relatively superior.
The most obvious issue is the price. Price on gaming laptops are usually determined by performance-weight ratio, or performance-battery life-weight ratio in the case of laptops with Nvidia Optimus. On a same config, the bigger and heavier the machine, the easier it becomes to cool so it's cheaper to produce.
Currently, the lightest rigs are Gigabyte's P3X, Aero, Aorus lineups, MSI's GS series, ASUS' GLX01 series and Razer Blade. On average, the weighting are as follows: 17.3" - 1080 - 3.2 kg, 17.3" - 1070 - 2.7 kg, 15.6" - 1070 - 2.5 kg, 15.6" - 1060 - 2 kg, 14" - 1060 - 1.8 kg, and 14" - 1050 Ti - 1.7 kg. And these numbers are going to get lower when they finish adopting the Max-Q design. Compared to this, the Alienware 17 R4 (1080/1070) is 4.5 kg, 15 R3 (1070/1060) is 3.6 kg, and 13 R3 (1060/1050 Ti) is 2.5 kg. That's about a kilogram more for every display-dGPU size category, and you pay almost the same amount or just slightly less for more weight.
I'm not a fan of bulky and heavy laptop as I rarely commute on foot but rather prefer to carry a mini form-factor gaming desktop if needed. On a daily basis though, I use a lightweight gaming or workstation laptop with 8~10 hour battery life because I don't want a desktop or any other peripheral PC.
Having said all that, reviews including NBCs' and user forums all tell me that Alienware's performance is garbage, which isn't surprising at all. QC is one of the worst too and under Dell's management, it's actually gotten worse as opposed to what some people may claim. Performance and quality wise, I'd rate MSI>Clevo>Gigabyte(+Aorus)>Asus>Acer>Dell Alienware, without counting Razer because I won't buy a quad-core laptop with soldered ram.
So really, the only advantages Alienware have are the relatively durable build quality and the unique combination of large batteries with Nvidia Optimus and some people say a better keyboard, but I disagree on that. The problem though, is if I wanted all that while sacrificing some game-specific features like G-Sync and 120Hz Panel, I have Lenovo Thinkpad P, Dell Precision 7000, and HP Zbook lineups that are much better in all those areas, with extra perks like business security, more storage bays and ports, pen-touch displays, oh and also being a kilogram lighter. And with mobile workstations, there're always "holiday+business" discounts that go up to 50% so that the price becomes pretty justifiable for an average consumer.
So other than to get that OLED screen on the 13 R3, you see why I don't understand people who buy something so expensive and isn't worth the price tag.