I don't know much about Hatch peppers other than I think they are only available in the Southwest.  From what I just read, they are only about 1/3 as hot as a jalapeno, but can be as hot as a jalapeno.  I like spicy stuff so something less than a jalapeno really has no attraction for me.  Is there something special about the flavor that makes the Hatch popular or is there something else about them.  So a few posts on here recently mentioning them so figured I would start a thread to have some folks here educated those that don't know much about them like me.

I like them, they are versatile, but to me when roasted and canned, very mild.  Great in grilled cheese, beef burritos, egg salad.  I just like the flavor and texture.

What makes their flavor different than a bell pepper (green or red) or a jalapeno?

They typically are Anaheim Chilies and about 4-5 times the size of a Jalapeno.
They come in heat from mild to X-Hot.
I just got a 2 cases of them (roasted), one mild and one medium hot.
They have a good flavor, different that a Jalapeno, different that a Bell Pepper.
When roasted they are great for adding into a casserole, mac and cheese, or anything else.

We don't have Jalapenos in the house much, except when making ABT's.
Sometimes chop them fine and put them in Deviled Eggs.

Maybe the big attraction is size.
If you roasted same amount of Jalapenos it would take a lot longer taking heads off and skinning then after roasting and not much would be left.
More bang for the buck, so to speak.

What makes their flavor different than a bell pepper (green or red) or a jalapeno?

They come from an area in the Hatch Valley of New Mexico.
The flavor is something akin to how grapes get there flavor. The soil in the Hatch Valley is key part of the flavor for this chile. Heritage chile plants were used to develop the modern New Mexico chile peppers by horticulturist Fabián Garcia and his students, including Roy Nakayama, at what is now New Mexico State University in 1894.

Hope that helps somewhat answer your question?
mk

How would you describe the flavor?  Are they sweet, bitter, etc.?

Here is a link about them on Wikepedia.

https://en.wikipedia.org/wiki/New_Mexico_chile

A local grocery store here brought some in and roasted them on site. Heat level ranged from mild to very hot. The ones I bought were much hotter than most jalapenos.

Have you ever had Ortaga Green Chili's?

Have you ever had Ortaga Green Chili's?

not that i can recall

Buy some and you will know what a Hatch chili tastes like.

Or...  Lew, you can PM me your address and I will send you some that are already roasted, skinned, vacuum sealed and frozen.
You can try the real deal that way.

I think the Ortega canned chilies may have preservatives so taste would be somewhat different.
Not 100% sure on that, been a long time since I had them.

I actually ordered some fresh this year to try. When in Colorado a couple years ago, I had a few Hatch Chile dishes at a couple of restaurants and really liked them. My understanding is they are similar to an Anaheim Pepper and the size/type of Chile that Chiles Relleno are made from. Like other mentioned, it's more of a texture/flavor thing than just raw heat. I used canned diced chiles some in a few dishes and plan to start there. I've seen people say they are great in big slices on cheeseburgers too. 

Green chile cheeseburger is my favorite guilty pleasure. The fresh roasted is much better than the canned.  I would suggest Bueno frozen green and red chile if available.  When I lived in Santa Fe the Big Jim chilies from Hatch was what everyone talked about.  The dried red chili from Chimayo is also very good if you can find the real thing.
Thanks for reminding me it's time to load the freezer with green chili.    

Green chile cheeseburger is my favorite guilty pleasure. The fresh roasted is much better than the canned.  I would suggest Bueno frozen green and red chile if available.  When I lived in Santa Fe the Big Jim chilies from Hatch was what everyone talked about.  The dried red chili from Chimayo is also very good if you can find the real thing.
Thanks for reminding me it's time to load the freezer with green chili.   

Hard to find a burger joint in New Mexico that doesn't have Green Chili Cheeseburger on the menu.
Pizza places have them as a topping as well.
The Bueno frozen green are pretty good. Never tried the Red.
Early after the season you can find whole roasted frozen chilies in some markets, but they don't last long.

Just bought some of this today

https://www.dietzandwatson.com/product/Turkey-Breast-Hatch-Green-Chile