Your going to have to elaborate much more on what exactly your looking to do. Running vanilla is the 'lightest' you can run a server. The more plugins you use, the higher the chances of the 'weight' increasing...but its also subject to which plugins. How about listing your server specs, average player base, and overall plugins you use. Then we can go from there.
Yep. It's quite fast, but incomplete. I have high hopes for it- in fact, I hope if it succeeds, that EvilSeph will merge Bukkit and Glowstone for faster development. Sounds odd as Glowstone already implements Bukkit, but I think building Bukkit specifically for Glowstone would make for a much faster process.
I've been on the hunt for any server alternatives and keeping an eye on them all the time. Don't know how this one totally blew over. There was a mention months ago a few high-profile dev's were working on a C+ MC server. Haven't heard anything since.... Would love to see an MC server that was more efficient in the backend.