In TTRPGs the leviathan is DnD, or its bastard offspring Pathfinder. Of course the competition I am thinking about right now is Heroes By Trade and I am considering what I would say if I had just a few seconds to get people's attention. The key of course is to find a way in which DnD sucks and HBT rocks so that I can set up the most favourable comparison possible. Just saying "HBT lets you play fun fantasy campaigns!" is useless because plenty of systems do that; I have to showcase why and where HBT is better.
Complex choices, simple math
Interesting tactics for all classes and roles
You can actually play out Lord of the Rings
Worlds that make sense
These are some things I have been thinking of. The first two are huge because I always hated the situation where it was blindingly obvious what to do every round but actually calculating the results of the optimal choice took forever. "What is my plus to hit on my secondary offhand attack roll again?" Figuring out what your bonus to hit is should *not* take five minutes. I think that is actually a huge draw for new players because I often find newer folks absolutely floored by the task of keeping track of their current numbers and veterans who are bored because their tactics almost all come down to "I attack". There is a reason that nearly all the indie games out there that get any traction have extremely simple minimalist rulesets.
The latter two phrases are most important for the GMs I think. I always found it frustrating to come up with worlds that didn't fall apart when high level spellcasters began to do things - the amount of work it took to sort out exactly how everyone was supposed to defend against powerful PC spellcasters with clever players was prohibitive. I also really have a personal thing about healing and recovery; I despise the DnD instant heal system that makes injury a trinary system. Either you are full, hurt, or dead, and there really isn't anything more to it than that. Fantasy stories just don't work that way!
I figure it is important to keep these things in mind when designing a new game. It can be fun to build something to compete against the leviathan but if you actually want to see your game played you should figure out what other games are doing badly and then do that really well. It is going to provide a place for players who really want that and it will make a great marketing pitch should the time to sell ever materialize.