:-) You must not be a KSP player.
You got me there, I don't too well in sandbox-ish kind of games :)
Some of the 'issues' were:
Many mods didn't work any longer, or didn't quite work right. Fixes and updates are being/have been issued but that takes time.
That's on the mod makers, nothing we can do about that :)
Several spaceship/aircraft parts were updated and/or moved to a different place in the parts tree. So parts that were unlocked and available in an ongoing science or career based game were no longer unlocked in the new game.
Continuing an ongoing game wasn't always possible because your existing craft wouldn't work right, or parts they needed weren't available since they weren't yet 'unlocked' in the new version.
These kind of things happen as devs tend to change their mind about things, luckily you can always ask us for an older installer if you really need it :)
Since I wasn't that far along in the game, my response was to just start over again from the beginning. That seemed to be the general approach taken by most. Many people seemed to keep a copy of the older version around to play in parallel with the new one until they finished migrating stuff over, their favorite mods caught up etc.
That's why we provide offline installers so that people can back them up if they so choose :)