* Liadan drops a couple trias in the bucket
Kronwag, in my experience, the difference between /tell and /say really falls onto who you're interacting with. What i've found is that most people do use the brackets to denote what is OOC and what isn't for both ways of communicating. Some, i've even found, drop the brackets when it's a long-distance /tell (ie I'm in Hydlaa and they're en route to BD), since there is no way it could be confused between IC conversation. Tells are also useful when you don't want the rest of the populace to know what your characters are doing in the upstair rooms of Kada-Els.
As you can tell by the multiple answers, people have different opinions as to how and what should be used in what circumstance. The best thing you can do, is adopt or create your own standard for using them, then depending on who you're talking with, adapt your standard.