Multiplayer

"Quickly, code that last arena. We'll shove it on the shelf now, forget it if it lags"

Uh oh, the dieing words of a game.
Yes alot of developers, not usually indie, feel that multiplayer is an essiential aspect. And quickly add it, without thinking of it carefully.
Alot of players want to just play a game, then wait for the sequel. They don't want to complete it, then waste more time on multiplayer.

But don't ignore multiplayer, add it - but make it good.

Pro Evolution Soccer suffers from terrible online lag. Something you definetly need to avoid.
And make sure multiplayer is fun not just chucked in at last minute.
And finally, make sure you think about it carefully. Add a variety of options, maps and more!

Well that's all really, think carefully!