It's more of 'is this user important or not', a lot of people are serious.

Serious / important people:
conspirosphere.tk
PsychoticBoy - runs ABM
usagi - ran GLBSE funds with questionable practices, silver asset is pretty legit
TradeFortress - btcinvest looks promising
Deprived - very active in the securities section with high quality contributions
MPOE-PR frequently trolling but you're stupid if you ignore her
Namworld - runs various high quality assets (except BTCMINING which loaned out their coins during the period where loans were like the mortgage bubble)

Totally ignore because troll
Monster Tent - "P.S. In almost all cases where someone uses a woman as their forum avatar they turn out to be a scammer. " LOL

Just irrelevant (/ not important)
The people here aren't people you should ignore - a lot of people here trade bitcoins, buys assets, et cetra but they're not really people I consider important.

Tritonio
marketersales - asks for loans and lawns straight out of newbie queue
regular
camolist - not important
stochastic - lots of posts, doesn't matter
viboracecata
Bitinvestor
dishwara - not important, he did run the DISHWARA asset in a pretty joke style

Undetermined
WinTame2012