At the moment there is an issue causing many players trying out multiplayer for the first time to leave. Basically, those that play multiplayer the most are also usually the most experienced players, and so when a new player decides to try out multiplayer they will probably be up against someone significantly more skilled than them. Repeatedly losing to someone while they take almost no damage is obviously discouraging, and often causes said new players to either not play multiplayer anymore or stick to only creative mode games.
This creates a "death spiral" where because new players leave when they try multiplayer there are fewer new/ less experienced players for other new players to fight against, causing them to leave. This, I think, is one reason why you often see a lobby full of creative games and only one elimination game being hosted by an experienced player, with maybe a DOM game (cause few competitive players play it).
Luckily, assuming lots of new people buy the game on Steam when it releases this problem will be fixed due to the sudden influx of players after release. The bad news is that eventually, the numbers will die down and the problem will resurface as some of those new players become skilled players of their own, and others lose interest.
So, the game needs a way for players to be matched against other players around their skill level. My suggestion is a matchmaking system a lot like other games, with a division between ranked and unranked (casual) play, and a simple Elo rating. I'm not going to suggest anything about how it should be implemented, but if you have any ideas this thread is the place to discuss them.