The open beta for MechWarrior Online was initially planned to launch today, but due to client instability and subsequent community concern, it's been pushed back to a currently undetermined date.
“We have decided to push Open Beta back,” says Piranha Games' lead designer Paul Inouye. “We agree that the game’s current state of stability is not allowing us to get the latest experience across to new and veteran players alike. This is a short push back on the date and will depend on the stability and playability of the build.”
Among the issues that will be addressed or have been already: lag, matchmaking, and balance.
So while it's a bummer and perhaps a bit of a worry, a little patience should mean a more polished experience overall, and a more accurate representation of what the final version of the game will run like.