I haven't participated in a full one yet, but there are several things I like about it:
1. It encourages intra-realm PvP.It's always a thrill to see a name you know in a battleground, but as Megan noted, Wintergrasp takes this to a whole new (old) level. One of my guildmates mentioned that so far on Tuesdays, a lot of the top raiding guilds on each faction rally their troops and head to Wintergrasp to seize control. When this happens, real rivalries develop. It's putting the world back in world PvP.
2. It means something.If your faction controls Wintergrasp, it has a noticeable affect on the world. Your faction gets Stone Keeper Shards in instances, people gain more experience from kills, and you have the option to raid the Vault of Archavon. The first two are Northrend-wide effects. As far as previous "world PvP" iterations, they were local zone buffs. As for the Archavon-accessibility, the dude is a single boss encounter that drops t7 quality epics. Take that in - run Naxx for a few hours and get t7... pop into Archavon for 15 minutes and get t7... I'll take it!
3. It's not yo mama's PvP.Destructable walls! Siege engines and gunners! And, lo and behold, occasionally organized strategy. In WSG, EotS, and AB, you couldn't really change the world around you at all. It was all "kill them other fools who be tryin' to do da bad ting!", but for the most part, it was just flag-tag. AV had a different slant with capturable strategic objectives, but at the heart of it, it was flag-tag with a PvE tank-and-spank boss encounter at the end. Wintergrasp though is an entirely different matter. Change is good.
4. Strategery beats superior numbers.Yes I meant to spell it strategery. No I don't care if the real word is strategy. It isn't perfectly balanced yet, but because of the tenacity buff, one side can't (always) simply zerg with more people to win. There are tactical points on the map that really matter if you're holding, and there are choke-points, and foot soldiers protecting valuable siege engines... glorious!
The one thing I dislike? The end is just a frenzied zerg in the courtyard. I was literally standing there, Consecrating and Divine Storm'ing in a sea of names, hoping I killed things.
I'd definitely go back, though. Once I accomplish a few of my PvE goals, I'll be keeping an eye on the Wintergrasp timer and participating in a full one to get the "real" experience.