This is the problem with these pages and pages of arguments. It comes down to language being a weird and fluid thing.
Would I consider Dawnguard a micro-transaction? Technically, it is. Sure. It's additional content, offered in addition to the base product, at a lower price. Technically, I can't say "No that's not a micro-transaction".
Colloquially however, I wouldn't call it that. To me, and to many gamers I know and talk to, it's an expansion.. Or would more likely be called DLC, or downloadable content today. (Which adds another sticky layer to this doesn't it.. Is all DLC Micro-transactions? Are all Micro-Transactions DLC?)
While yes, I have to concede that it's technically a micro-transaction, it's really more than that. It's more, additional content. It's more game. To continue the fast food metaphor others have used in this thread.. It's like you go and have combo meal for dinner, and you really enjoyed it, and are still hungry, so you go back up and get another order of french-fries.
I guess what I'm getting at is that there are tiers to micro-transactions. These are usually considered "Good" micro-transactions. The next tier down is cosmetic. These are small things that really offer no benefit to the actual gameplay, but are just little niceties. Skins in a game, or different dice in DDB. Gamers typically accept these as one of those "Cost of Doing Business things". We know game companies want to increase revenue, and we're okay with them offering these things as non-invasive methods. These get less acceptable as they start to provide game-play benefits. Like the ability to purchase level-ups, or stronger equipment.
I don't think D&D can have this problem due to how the game actually works. Like as a player I'm allowed to make a level five character instead of a level one character if I want. There's nothing stopping me from doing so. On the flip side DMs/Groups get to police their own tables. Just because I show up with my level twenty character decked out in all the cool gear, doesn't mean you have to let me play it. I don't think you can really sell these things.
The only reason I worry about micro-transactions is because they(The general "They" as in corporations..) only ever take things away, and then sell them back. Even looking at the non-invasive Micro-Transactions like skins that we consider normal these days.. That used to just be part of a game. Halo 3 had all the cool spartan armor in the base game, and you unlocked it through play. In modern halo you have to swipe your card for it.
In DDB you're currently allowed what, 10 characters for free? In a few more years is that going to go down? What if they lock the character builder behind a paywall?
Those are the things I worry about. Like other's have mentioned, they have no obligation to provide these things to use free of charge. We're lucky to have them as options. But that doesn't make it not-suck when those options are taken away.