It's not an underpowered laptop, it has one of the best mobile processors available today and it has very capable graphics for 1440x900, and depending on the game you may be able to run it at 1920x1200 or 2880x1800 with acceptable frame rates and settings.
What model is your Alienware? It's a gaming rig, not an all-rounded system. It would be heavier, thicker, and it would have less battery life. Then of course it has the lower resolution, and unless you upgraded it no SSD either. .
For me, yes, it is extremely underpowered, its GPU is fail. I deal with pretty intense simulation software, and my alienware can barely handle it. ( hence the 20K workstation at work )
I have an Alienware M18X, picked it up a few months ago, I can't run as high as a rMBP, I'll admit that, but I am willing to trade lower res for a much larger screen ( 18 inches ) and better performance. ( 4.1ghz i7 overclocked, 32gb ram, 768gb SSDs in a RAID, I have dual SLI 7970 MObile GPUs ). Now that might sound stupid, but I use every bit of that performance for work, and play.
So yes, to me the rMBP is underpowered,
Yes, my laptop weighs about 12 pounds, but cmon, what is 12 pounds? If 12 pounds is alot of weight for you. Time to get the gym and loose that 300 lbs of body fat

I'll take my 18 inch screen and hardware performance over underpowered tablet Laptop.
It would be heavier, thicker, and it would have less battery life. Then of course it has the lower resolution, and unless you upgraded it no SSD either.
Other than the Low res, its better than a rMBP.
Less Battery Life? Wrong, I can swap my batteries out, and I carry 3 with me in the field.
Heavier and thicker? Yes. That doesn't matter to me. I'll take the performance. 12 pounds is nothing. Gym it up.
I have 3 SSD's.
Yeah but again the rMBP is cheaper than the sMBP with the same specs. I see an iMac with Retina display being the same.
I can't see Retina in the iMac yet, I think even with Apples buying power, to expensive.
If I were to buy a Apple laptop, it would be a refurb 17 inch. I like screen space. ( The last Apple laptop I bought new was a G4 Powerbook, before that, a Pismo that got me through college )
I say it's a waste of space because it doesn't get used that often but still takes up a significant amount of space, particularly in a laptop. (And I believed this years ago, because I haven't really used an optical drive much at all in that time.)
For me., it isn't Because I use my computer for work. Not play.
It is subjective though, for all I know you're using the optical drive in your laptop for three hours every day. I've probably only needed to use my optical drive four to six times this year -- two or three for installing Windows 7, and the other two or three for installing Battlefield 2142. That's it.
Yes it is, your 100% correct on that, but in something like an IMac, space makes zero difference, even if I only used them a dozen times a year, I don't want an external drive.
It's better the laptop is lighter and has a thinner profile, and then those who need optical drives can buy an external one.
Opinion imo, Apple makes some great laptops for sure, but with the rMBP I see them mostly as toys.
But hey its all about what you need? With the rMBP I think its insane to kill the ODD, and ethernet for 3/16th of an inch.
I'll stick with my big Dell.
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I noticed that is how my comment was read and corrected it moments ago, when I say digital I mean digital downloads.
I know DVDs and Blu-ray are digital on discs, with how things are talked about these days saying digital tends to mean downloads from what I seen.
Blu Rays are still digital, so are DVDs
Try a Blu Ray next to Streamed content, the Blu Ray will blow it away.