Rechan... I will so totally play in your campaign.
Online games are tough. They certainly have benefits; they're convenient for those with a busy schedule or little money and can't deal with travel time each with week. They allow you to do some creative things with maps, artwork, music, etc., with the Internet at your fingertips like that. Online tabletops, if used right, can actually speed up gameplay, allowing you to have more game per session.
But... the hardest part is definitely getting a group together, and keeping it together. For me, as a player, the toughest part is finding a DM! And, on the rare occassions that I do, I often only get one session, if even that, before it falls apart.
Here's some things I've found that help; call it the habits of highly successful online campaigns.
1. Encourage interaction outside of your official play-time. All the online groups I've been with that lasted longer than a week had this in common. We had an online discussion board, an Obsidian Portal, or even just a chain of emails that we used to communicate between sessions. These ranged from "sorry, can't make it next week," messages to in-depth roleplay scenes to us just talking about how cool the last session was. It lets everybody get to know each other better. One of the problems with online games is that everybody feels anonymous, so they don't feel particularly guilty if they just stop coming unnanounced. If you can build relationships with your players, they're more likely to treat you like a real person. Go figure, I know.
2. Play-by-post is death. I've seen you post on the Maptools forums in the past, Rechan, so I don't think this is your particular issue. And it might be different with other systems outside of D&D 4e, I don't know (there's a rich tradition of PbP on the OotS boards, for instance, where 3.5 is generally preferred). But for 4e, play-by-post just murders immersion. Especially in combat, which already runs the danger of taking too long. Can you imagine a 4e game wherein players didn't take just minutes deciding their moves, but days? *shudders at the horror* For 4e, at least, online tabletops like Maptools are definetly the way to go.
3. Be (excessively) courteous. In the online world, where most communication is through text and text alone, it can be easy to misunderstand or misconstrue different messages. Also, since the Internet offers some degree anonymity, people feel like they have license to be a jerk if they've received (or believe they received) jerky treatment in turn.
What does this have to do with your game? Well, let's say, hypothetically, somebody in your game wants to use a Paragon Path printed in Dragon that you personally think is overpowered and/or doesn't fit your setting. The player asks via email if he can use it, and you respond, "No, it's overpowered." Since he can't hear your tone of voice in text, or see your casual, "Sorry, dude" shrug, he might interpret your response as draconian or just snippy and rude. He might get offended, and decide to stop playing. And, boom, suddenly you have a guy who doesn't show up to your sessions anymore.
The better reponse is to say, "Sorry, I think that PP is a little overpowered. What do you like about it? The flavor? The mechanics? Can I help you find something like this, but that won't break the game?" Basically, make your intentions clear at all times, and always explain the reasoning behind your decisions. And finally, listen to your players. I once had a DM who wanted to nerf my Swordmage's Warding class feature. I talked him out of it, and he let me play it as written, and I was really gratefully to him for that.
4. Check in with your players. The best online campaign I ever played in was run by a guy with the handle of Lordsmoothe (or something like that) on the OotS boards. We played on Maptool, with occassional roleplay on the OotS forums themselves. We didn't make it past level three, but we played for a solid month or so together, and we had a great time.
Lordsmoothe was a great DM. He tweaked an awful lot of stuff, but he always told us why he was doing it, explaining his reasoning. Sometimes, he'd even straight-up ask the players: "Is it cool if I use the Obsidian Skill Challenge system from EN World, or do you want to do it by the book?"
I think there's a certain arrogance that comes with online players. These players figure: "Hey, if this DM doesn't run the game I want to play, I can always leave and find a new game. It's a wide, wide Interwebs out there." So, naturally, once a DM starts slamming down the house rules and banning whole books, or starts telling a player to stop distracting from the main plot with all his fluffy roleplaying, or tells the power-gamer that he can't use his overpowered 3rd-party class or race, or whatever he does to ruffle an individual player's feathers... well, that player will be very tempted to leave.
I'm not saying you have to cater to your players' every whim. You just have to be aware that these players are not 'stuck with you', and any argument that relies on the notion that you're the only DM in town is doomed to fall on deaf ears. You still have the authority to say no, to ban disruptive players, to railroad when necessary, and generally do what you have to do.
I'm just saying... do it nicely. Oh, and most important of all...
5. Be a good DM. Just do your job, and do it well. Make cool plots and interesting encounters. You might lose some players, but the good players, the loyal ones, will stick with you and help you recruit more. It'll get easier as you go along. Many campaigns fall apart at level 1. The fact that you've gotten even a few sessions under your belt speaks volumes about your skill as a DM.
Keep in mind that when you're starting an online campaign, you're effectively scooping up six random people, putting them in a room together, and expecting them to get along and have fun. Unsurprisingly, this doesn't always work out. If you lose a player or two, but still have four people in that room having a good time, I'd call that a win.
Like I said, if you do your job, word will travel, and your players will be invested enough to help you find replacements. You'll find it easier and easier to replace lost players (see first sentence of this post), and then eventually not necessary at all.
