Exactly what it says. (Highlander means no more than one copy of any given card per deck.)
We all know that Legacy contains some of the most fun cards ever printed... But we also know it has a dark side: NPE and ridiculously powerful combinations.
Highlander, by and large, addresses these issues, by making it much less likely that overpowered combinations are found. Players may also find a need to play more attacks than usual, as they can't count on getting their recursion/discard-tutors out reliably. Deck construction requires a lot more thought, and the games play a bit more like they did back in Set 1.
I recently put together some 9 or 10 highlander decks, and I'm really pleased. I managed to maintain a lot of theme-y-ness. They weren't great to start off with, and they're all worse in highlander form, but since they were playing against each other, the games were surprisingly well-balanced. I think they may do okay in a highlander tournament, but I want folks to build some decks and give me some competition.
The only downside I'm aware of? Remy sucks used cowbell in highlander.
Will you join my campaign? It only takes a simple pledge:
"If I run one or more legacy tournaments each month, I will make at least one of them a Highlander tournament each month. If I don't run tournaments, I will pressure my scout to run Highlander legacy tournaments."
Even if you disagree, please share your thoughts.