I think PBBGs have a lot of potential, but like you say, most don't live up to it. The slickest (games like Travian) are mostly about designing the game to make money and encourage 24/7 play (neither of which make the game very fun for me). The majority (games like Torn) are not really interesting intellectually or sensorily (though I read that game made its developer a million pounds!).
Now, 'that said', I do like playing even stupid PBBGs for a brief time (until I lose interest). I lose interest because those games are so blatantly repetitive, and at the same time encourage extreme repetition (like having around-the-clock monitoring of your account and so on). It gets pretty mind-numbing, to say nothing of soul-crushing.
Why do I play them at all?
First of all they're very easy to get into, so it takes nothing to try it out. So, I would say make the game easy to play. I'm a fan of RPGs, strategy games, sci-fi, swords and sorcery, espionage, survival games, etcetera, so I enjoy any setting, theme, or mechanic...so my advice here would be don't be afraid to choose a niche setting, genre, or theme for your game. The web is so accessible you are likely to find players where you wouldn't on a less-accessible format (even a simple download puts some people off!).
Third, I would say balance this accessibility by not making the game such a clickfest, or dependent on investing massive amounts of time...I think this kind of design ruins the fun. Obviously if you're making money on the game...you may think otherwise!
Finally -- so many of these games just look horrendous visually. Not speaking in terms of graphics (many of these games are mostly text) but just basic typography and layout. Put some effort into visual polish and you'll rise above the pack.
I think PBBGs are great for building a fun community around a game because the web is so accessible, you can get a great critical mass of players. Concentrate on building a positive, healthy community and you can get something really fun to play.
