Clockwork and Dark Harvest were both month-long events, if I recall correctly.
As stated, Clockwork revolved around a galactic-scale AI named Omni that created a massive droid rebellion and was able to 'infect' (for lack of a better phrase) sentients to use as agents for its own goals (wiping out organics, taking over, etc.) The major criticism for it was that, well, there weren't enough people running it. Most of the time the members had to just run everything themselves because we hadn't planned for just how much activity was going to skyrocket for it.
Overall, however, Clockwork was great and had a ton of great story. It is, for example, the reason that the Protectorate doesn't trust AI's and installed so much security on Fondor - Omni was basically able to take over the planet and they want to keep that from ever happening again.
Dark Harvest was last Halloween, and involved a lot of zombies and multiple threads in a manner similar to Masks of Madness. You had your PvE threads, your PvP threads, etc. Rewards were given out, typically at random, to those who participated. I, for example, received stealth armor. There was also an event ship or two given out.
The major criticism there was... well, honestly I can't remember, lawl. Someone else will, I'm sure. It likely had something to do with rewards, or that you originally could only join (I believe) one thread out of the three. The threads took place on multiple planets before the finale on Dagobah.
Dagobah itself was a Charlie Foxtrot. There was a massive, massive space battle and attacks on a moon base. HK went onto the ground through a blockade and go his entire unit wiped out. All sorts of things went down - like Velok being Velok and Sarge touching artifacts that he shouldn't.
Fun stuff, fun stuff.