This is the dude that is the head designer of Magic: The Gathering. Depending on who you ask, he is a dedicated game designer who consistently delivers interesting designs and fun new sets. While he does keep stuff classified until the public is supposed to know about it, he considers keeping the secrets the worst part of his job. The card line Maro is named after him (MaRo is an alias of him). He likes mechanics that double stuff, and guesses that 75% of magic cards that fit this category are designed by him. He considers his biggest contributions to magic are the psychographic profiles and giving lots of things names. The first is a tool to help R&D appeal to a diverse player base and helps the community understand each other. The other gets people to talk about stuff because there are words for it. As for his psychographic profile, he is a Johnny. He claims to be a big believer in "restrictions breed creativity", meaning that people find creative ways to work around restrictions. This one of many reasons he likes the color pie. If you ask him to sign one of your cards, he will probably sign it, but be aware he will probably draw on it too (also be aware he has freely admitted that Look at Me I'm the DCI is an accurate demonstration of his skill at drawing, best out of 60). He can't pronounce "Rakshasa", so a fun game at WotC is to try to make him say it. Because he is the most public figure in Magic and the highest ranking member of a section in R&D (formerly Design, which some people assumed was all of design, when it is basically the the first half. After R&D was restructured when making the return to Dominaria, he became the head of Vision Design.), he receives the blame for most things people dislike, sometimes wrapped in vitriol. He estimates that 90% of the time he had nothing to do with the grievance.
Cards He Hates
If the card does something outside its part of the Color Pie, there is a decent chance it belongs on this list. He is a Big fan of the Color Pie. Citations appreciated.
- Leeches: Because this makes poison counters just "another life total"
- Hornet sting: This card fills a niche in green MaRo thinks should be left empty or handled differently. In terms of how many people ask him to sign a card, this one is in 3rd (the first 2 being Maro and Look at Me, I'm the DCI), because people think it is funny to have him sign cards he doesn't like.
- Beast Within, Song of the Dryads and Lignify all allow green to effectively destroy an opponent's creature without using creatures. 
- Chaos Warp allows red to turn anything into a random card from its owners deck. This allows red to destroy enchantments which are suppose to be one of its weaknesses 
- Path to Exile: This card is removal that white is able to perform without being provoked (here is a good example of what he means by provoked ). He feels white locking a creature down unprovoked (e.g. Arrest) is okay though. He holds a special hatred for path to exile because it is a tournament staple. 
Cards He likes
- Maro, a card he made and is named after an alias of him. Owns the card art for this card.
- Look at Me, I'm the DCI. He made 60 versions of the art for this card and the one he liked the best is the one that got used. Got paid a dollar for it (WotC insisted it had to pay him for legal reasons or something), insisted that the dollar be in check form (even though the check is more expensive than a dollar), and framed and mounted said check on a wall in his house. Owns the card art for this card.
- Doubling Season A doubling card that's one his personal favorites. 
He considers his favorite format Magic Design play testing. There is no art, card names are often silly place holders (for example Trepanation Blade used to be called Chainsaw), and it is common for players to change the cards stats in the middle of a game (this last thing is less common for people not actually part of Magic Design staff, e.g. focus groups). Also the intent is to figure out what's fun, not trying to build & pilot the most optimal deck. Also Maro says "we play a lot of bad Magic so you don't have to" because sometimes, especially early on, the mechanics are not fun because it's not the refined and finished product they put out. It's certainly not something for everyone.
The Storm Scale
Rosewater thought up a scale to rank the likelihood of specific Magic cards, settings, and keyword mechanics returning in standard legal sets. It's named for the infamously broken storm mechanic, which he ranks as having no chance whatsoever of being used as a major theme ever again. The scale uses ranks between 1 and 10, although oddly, things that are less likely to return are given higher numbers. Probably it should really be a ranking of something's potential disruptiveness in future sets, but Rosewater's background isn't in a technical field so he probably just never thought about it.
Rosewater regularly communicates with the public via social media, mainly Tumblr. You can submit questions and comments on sets, cards, design, and random stuff, though you usually have a much better chance of getting yours addressed if you phrase it in the style of "Why are you so awesome?". Also if your username is NSFW then he won't answer you, because his answers are for everyone to see (including those that aren't old enough to be exposed to said user name).
Do you have an opinion on Dredge? Well, this guy is responsible for a lot of that deck. We don't mean he built the deck, just that he designed the cards (or contributed to the design of these cards) that ended up in the deck. This includes (but isn't necessarily limited):
- Dread Return
- Bridge from Below
- The literal dredge keyword ability (admittedly his version was just replacing card drawing with getting a card back from the graveyard, the addition of also milling was something development added).