What I mean is the associations with the name of a product which differentiate it from a generic version, in the eyes of the public*. One of the reasons I think D&D has an undeniably strong brand is that it is regarded as the only example.
Name recognition by itself doesn't mean much in this case. What matters is WotC's ability to translate it to making money for them and not somebody else, and that is where they have generally fallen well short. Even with the successes in the other markets, they probably haven't seen that much of the profits. It's almost like there is two D&D brands. One is the formal brand controlled by WotC (and TSR before that), and one is the overarching fantasy generic brand that covers not only most tabletop games, but a fair number of different novel writers as well. The latter is extremely strong; the former has never done nearly as well. That's the factor that most of the comparisons to Marvel doesn't take into account. For most comics, there is virtually no gap between the popular usage of the brand and the actual formal brand. For D&D, that gap is huge.
WotC has virtually no control over and makes virtually no money off of the more generic version of the brand, and it's strong enough that they can't squash it either. That makes it harder for them to sell licenses to the formal brand. There's little incentive to pay for a license from WotC when one get most of the elements from other sources and put together something that is visibly indistinguishable from D&D, but legally and financially is very, very different. For the cost of what they would have paid for the license, many companies can mount an effective PR campaign to overcome the lack of name recognition, breaking even in the short run and ending up with an IP they have full control over in the long run. Add in a history of being a difficult business to partner with, and WotC has even less clout to sell formal licenses.
In the tabletop market, they could get away with writing licenses that don't give everything away if they took the time to build good relationships; the indy publishers can easily create sellable content at a low cost, so they can get away with more limited returns. With movies or video games, it's a different story; a lot more money can be made, but they also take a lot more money to create in the first place, which the licensee is going to expect to make back and then some in equally large proportions, so WotC isn't actually going to make that much more than if they spent the money directly on their own product.