You ask a tough question, so I'll try to throw out an acceptable answer.
Of the Pennsylvania based cons to compare this one too that are currently active with back to back years you have :
(Erie Anime Experience really can't be compared as they missed 2008)
If you compare this con to Tekkoshocon, Tekkoshocon is your large small/small mid-sized con with 3000+ attendees. With the move to the David L Lawrence Convention Center, it's got a big con feeling. The building is quite comparable in size to the one at Otakon. Huge. Stuff going on all the time pretty much, lots of roaming space. panels, gaming, etc.
If you compare to Setsucon, Setsucon is like Zenkaikon was in 2007. Small, very centralized in one space, very personable/interactive with other guests. Had around 500 guests. Lots of stuff to do, but not really around the clock and it was all in one place to a certain point.
Now the funny thing is, its going to be hard to compare Zenkaikon 2008 to the upcoming 2009 because of the move to the other part of the hotel/convention center, but I'll take a stab it it. If you mixed Tekko and Setsu, you would get Zenka. It's right in the middle. Last year more space was opened up, but it still had a very nice small/personable feel to it, though it really was not small (1352 attendees). Zenkaikon also had some really really good panels, very comparable to the quality of the ones at Tekkoshocon (possibly better in come cases). It was a tad crowded at some points last year, so hopefully the move will resolve that. Also as a note, food access is not always the best, so bring money for Pizza and Chinese order-in's, unless you can get to the King of Prussia mall.
To end my babble (XD), I'll throw in some personal opinion: The staff are really cool at this con, a few of us came down from our Anime Club in 2007 just to see what was up and came back in 2008 with most of the club. It's a really well run con, and I look forward to coming back. Have not had a bad time yet. Hopefully this helped.