* On one hand it's one place to get all your professionally designed skins, texture packs, add-ons, etc. for Win10/PE, Mobile, and shortly Xbox. Also, you don't have to play old versions of those games for them to work. Finally, "modders" (actually "add-on makers") will get paid!
* On the other hand, note the word "professional", meaning the programmers of these things need to get paid in something other than a pat on the back. While some will be free, most will not. You'll have to shell out real money to buy MS "coins" to trade for those things.
When I saw the news about the Marketplace I searched in my PC Minecraft for it and was confused to not find it. Then I found it in the latest update of Minecraft Pocket Edition on my IPad. Now, instead of paying €6.99 for a Mojang mashup pack, I have to purchase Minecraft coins which I can use to purchase official Mojang add-ons or add-ons created by community members. It is nice to have the option to purchase community-created stuff in-game if I want.
I think that Mojang is being really clever about how they entice users to spend more money in the store. Example:
The Mojang Mashup packs (e.g. Fallout, Chinese, Greek, etc) all used to cost €6.99. Now the new Skyrim Mash-up pack costs 990 Minecraft coins. If I decide to purchase that, I have to purchase a package of 1020 coins for €6.99. I will have 30 coins left after purchasing the Skyrim pack. So actually, it has become cheaper, but what do I do with the 30 leftover coins? I think I would be more likely to make more purchases if I have such leftover coins, because I want to use them up.
Mojang has nothing to do with this. They only develop the java version.
This is pure Microsoft marketing.
One of the core principles, and reasons for the success, of Minecraft is the "buy it once, get FREE updates forever" - that's the way the "true Minecraft" (java/PC) has always been, and - hopefully - always will be.
Thanks for the correction! Being relatively new to Minecraft I wasn't around when Microsoft became involved and still don't fully understand what's going on. As a newbie I find the existence of so many different versions and the relationship between development and marketing to be very confusing.
What Herb_ said was almost all true, especially the "Now, hopefully, Microsoft won't mess things up. So far, maybe surprisingly, they really haven't. They realize the 'special nature' of our community, and they've almost entirely left Mojang alone, to carry on as they were."
Updates for all editions will still free, and there's a 99% chance this will remain so.
However, MS has two and a half billion reasons to change the business model some. As Cherry Samples says, the "Buy one pint of milk and get milk for life!" doesn't work for grocery stores either. A few things will still be free, but usually you'll have to pay a few bucks if you want to customize the game further. This is no different than buying DLC for other games.
You might want to check the article on the News page about the "Better Together" update that's coming. The name "Minecraft" will apply to all the editions getting that update, and the java edition (which isn't) will be called "Minecraft- Java Edition". Reading between the lines, my thinking is the "flagship" editions are changing hands.