What is special about 1x11 - why not just big cassette and single front ring? I currently ride a single speed belt Soma Wolverine with road bars, hydraulic disc brakes and I love it. Only trouble is my knees are telling me I need some lower gears especially up the hills. That's 55:22, 175mm crank, 700c-28 wheels - About 69 gear inches? Go to a 50-tooth front ring sure, but then I would go spin out even sooner down hills. I want to keep it simple, I have tried Alfine 11 and did not like it (leaks make the clean belt drive dirty, big heavy hub on the back wheel), do not have the money for Rohloff. Looks like a 1x11 setup is for me.

SRAM has 1x11 group sets that will do the trick. But SRAM is uncommon - never seen it on my bike trail in Melbourne - and discounts are far less common.

With other big two, fair chance of picking up discounts on random 11-speed stuff. So question is - what makes a 1x11 groupset special? Is there any reason not to pick up a heavily discounted Ultegra spider, a  third-party 42T ring, a Deore XT 11-42 cassette, matching XT derailleur and matching brakes/shift levers? 

The issue could be that moving from one extreme to another the chainlink is just a whole lot wider and the chainline way more bent than it might be designed for. Do 1x11 groupsets have any design features to deal with this? Do you need a chain designed to flex to allow for this worse chainline? I have no issues building wheels/changing bottom brackets to make this work.

Update: 1x11 is not so hard! Here's what worked for me.

Brakes: This is the driver for a lot of decisions. Wanted drop bars, hydraulic brakes and lever shifting. All brands have options but I chose Shimano Dura Ace levers and calipers, based on available deals. Also as mentioned much easier to build the whole kit with compatible, discounted parts. Unfortunately the Dura Ace calipers will not work on my IS-mount frame because there just is NO flat&lt;->IS adapter as there is for many other combinations. They're put away for the next frame.

Crankset: Dura Ace FC-R9100. Again, the big discount tempted me and it's really pretty - functionally probably no better than Ultegra. And I can sell the two rings it came with.

Front ring: Wolf Tooth 110 BCD Asymmetric 4-Bolt 38T, with their special chainring bolts - very nice, hex key on both sides! "Rides of Japan" on youtube showed a 1x11 conversion with this ring and Di2 system. Teeth are thick/thin alternating. You do have to file away a small part of one mount to fit it on modern Shimano cranks but is a small job, Wolf Tooth even explain this and the video shows it too.

Chain: SRAM PC1170. Comes with a special one-time link that makes joining pretty easy.

Rear cassette: SRAM PG1170 11-36 - very wide range, mid-price. I think the 42T would not work on the GS derailleur.

Rear derailleur: Shimano Ultegra RD-RX800. I worried about this because Shimano say this derailleur is good for a maximum 34 teeth but it handles 36 OK with plenty of adjustment left. Nicely made! This is the "GS" medium cage model with a clutch, so with the clutch engaged it will apply extra tension and stop the chain from slapping around over bumps.

Downshift cable hanger: A Wolverine frame has downtube shifter braze-ons. I needed to find one right-side cable hanger to make the gearshift cable run, or else I would have needed a full-length cable sheath.

Brakes: Going to use some Shimano M675 SLX calipers Was going to use some M675 SLX callipers but bleeding them I found one is leaking from the cracked ceramic piston and spares are not sold. Used for maybe 6 months commuting. Disappointing quality again, Shimano. Will now be using TRP Hylex (original) calipers with the with the Dura Ace levers. Have found people in forums successfully mixing Shimano calipers with Hylex levers so no problems anticipated.

Outcome: 11 gears between 27-91 inches. Both higher and lower than I had. Shifting is smooth.
SRAM have emphasized the 1x configuration, but 1x systems can be built from Shimano components too.

You are right that a 1x system is pretty much a wide ratio cassette and a single front ring. Double-wide chainrings are commonly used to help chain retention as there is no front derailleur cage to guide the chain. Clutch derailleurs are also used to tame chain slap on rougher trails encountered when gravel riding.

You can use a double crankset with a single ring on it, but it's a hack and as you say you will have cross-chaining problems. Dedicated 1x cranks align the ring with the middle of the cassette so the chain deflection is minimized.

You will not be able to use a XT MTB derailleur with a road drop bar shifter. Shimano MTB and road 11 speed use different derailleur actuation ratios (see here) so MTB derailleurs and road shifters are incompatible. The largest cassette that Shimano long cage road derailleurs can take is 11-34, but, if you are coming from single speed that is going to feel like a huge ratio range.