I agree with most of the responses so far, but I'm going to put in my plug for my favorite aspect of WIS...progressives.

To me, research and drafting are probably the most enjoyable thing on the site. Learning the history of the game, about players I've never heard of and building a winning team is satisfying on so many levels. That is why I had played in such a variety of theme leagues over the course of my WIS career. When I finally decided to bite the bullet and join a progressive, WIS was taken to a completely different level for me.

In a progressive, when you own your team, you truly own all aspects of it from both a managerial and GM level. You learn a tremendous amount of baseball history and about every player becasue you have to research them all for drafting purposes. You don't only learn about the 25 players on your roster, but every single player in the league as you have the ability/need to trade with other owners. The cookie strategy/seeing the same players over and over is obliterated, and every single player (even the guy with 200 PAs or the pitcher who slots in at Long B) are vitally important to your franchise.

The comraderie/respect/grudge matches with other owners is fantasitc becuase you face the same division rivals and other opponents year in and year out. You can learn their trends to better manage against them, develop trade partners and even make friends/rivals as the seasons go by.

You also have full control over your roster. You really want a player? Then pony up and trade for him. Have a weak postion? Draft a guy in the rookie draft who will fill it for a season or a decade. The choices are all yours to make.  Plus, becasue you are really drafting for this year, next year and the next 5-10 down the road, you are always drafting with the present and future in mind.

Also, for those who have stated the season is aticlimactic or a downer after you put together your team, then a progressive is a great way to break the mold. Very rarely in any progressive have I ever had the luxury of setting a roster and forgetting it, as the limited player pool often dictates a true managerial need. I've got 2 running right now where I think I've used at least 10-12 different lineups over the course of the season. Plus, you can make trades during the season, set matchups for division opponents and rotate your pitching staffs to ensure the best matchups.

I know a lot of owners are wary about joining progressives becasue of the long-term committment, but I think it is the best decision that I have made on WIS.

1. I agree that sim makes you a student of the game and increases your baseball knowledge. I can talk to my father now about players from the past that he used to tell me about, and now I know as much about them as he does. Just yesterday I was watching a show on the 1971 season on a baseball channel, and they mentioned Joe Torre hitting .363 that year, which was the highest BA a 3B had since 1930. They never mentioned who hit that high in 1930, but because I play the sim I knew, as many of you probably do, that the reference was to Freddy Lindstrom, who hit .379.
2. I heartily agree with frazzman. I picked up an abandoned team in a progressive about a month ago and then I joined my first progressive from the beginning a week or two after that. It is the most fun I have ever had. Far and away the most fun in sim is drafting for a progressive team. Give it a try if you have never done it. It's awesome.