It also tends to kill my interest to see the game consistently go in directions I don't like. Not much to be done about that, but there's not as much investment or point in playing when things keep going elsewhere from what you'd like.
...
I suspect there's also sometimes a sense that once a game is established, there's sort of an established set of situations or method of doing things or clique of decisionmakers, which makes an established game much different from a new one. People love popping into games to say "Turn into a potato!" or "Clowns everywhere!" for no particular reason. Figuring out which lead to investigate or what your next piece of dialogue should be is both more restricting and requires much more understanding of what's going on, which makes it less appealing to a lot of people.
In my experience, these two things tend to combine to cause playercount to drop over the course of the first few segments of a new game. When the game starts, there's a fair number of ways things can go, and in many games there's the freedom for it to really go
anywhere. The first few situations handled tend to establish what method and tone is most popular, and how the GM reacts to things this early goes a long way toward establishing that tone. Usually, this means the first major schism is between the silly lol-random people and the serious people, whichever camp loses usually quits the thread for another.
At the same time this is happening, there's usually a handful of very active players whose suggestions tend to be the most popular, which gives rise to the first clique arising. Once they get history on their side, there's a definite sense that "this is a sort of thing the character would do" which lends extra credibility to their suggestions, since it's hard to ignore internal consistency. From what I've seen, the less vocal players tend to either quit or start lurking, either because their votes can't overcome the clique or because their opinions are already adequately covered by the clique. This isn't necessarily a bad thing, since a small core of dedicated players will keep the game alive through various levels of popularity. This tends to be the point at which strong story wins out over strong mechanics, since people who are interested in the actual story of the game are more likely to hang around, even if they just lurk. People who hang around only for the mechanics will drift away if they get consistently outvoted.
Assuming the GM is still going strong at this point, what I've seen happen often is that the original clique will splinter based on smaller deviations than the broad strokes that brought them to the forefront in the first place. Depending on how things go, either history will repeat itself, or the splintering will allow the votes of newcomers to matter more, increasing player count. Once the game is established newcomers will have accepted the "story so far" as just part of the cost of joining the game, and internalized the history/motivations of the characters, meaning their suggestions usually fit within the broad strokes defined by the original clique. As far as I can see, this is how long-runners survive; new blood comes in at about the rate old blood loses interest in the game for whatever reason.
/pompous overanalysis
But anyway, as for your actual questions, I don't have much to offer other than what Fuzz and IronyOwl mentioned. Action/Combat that drags tends to kill things I otherwise like, and is usually a byproduct of adapting round-based tabletop rules to a setting where trying to advance the game in 6 second time increments makes everything take weeks IRL. I think the only GM I've seen get away with it is freeformscholar, and that's because he usually makes combat a series of quick updates that are done on the same day, or the day after.
I know irregular/infrequent updates kills it for some people, but I myself don't mind that much, and the fact I still have any players at all in
my games tells me there is a decent population of the forums that can live with one update every few weeks/months. I know that if infrequent updates are common, it can be difficult to keep up if the plot is reasonably intricate, which tends to drive away the more casual followers. I try to alleviate this by having every update linked from the OP for quick reference, where I don't reference the past directly in the update.
As far as joining new adventures vs joining established adventures, for me a lot of it comes down to subject matter and whether I'm already familiar with the GMs work. Blunderbuss is a pretty good example for me, actually- I remember the first time I saw it the game hadn't gotten very far (so I didn't know whether it was something I would be interested in a month down the line,) I wasn't in a super Steampunky mood, and I wasn't familiar with the you from other things. The second time I saw it I was jonesing for a good ISG, and not only was I more familiar with you from around the forums I was also quite aware you could deliver the goods on the "pretty pictures" front, which sucked me in long enough to decide the story was worth following.
I don't know about anyone else, but I don't spend too much time lurking around the main FG&R board looking for new threads. I keep involved in a couple-three at a time, and every once in a while I take a glance to see if anything catches my eye. This tends to be why I miss out on the ground floor of a lot of games, but if something looks like it might be interesting I'll read at least the first few updates before deciding whether to continue with it. Or bookmark it in case I'm not in the mood to read a wall of text at that moment- you know, like
this wall of text!