I use the Giant in the Playground site as well.
I can personally vouch for it in regards to being a great community to learn mechanics and discuss builds in.
As well as a great way to gather players for games on roll20.

Forum based though? That's not nearly as popular, so you might need to try a bit more with that.
But then again, I was part of a tried (and failed) minecraft server D&D game made there.

Just one note, don't go around talking about 3.5 pdf's and such.
I know they were legal for the past 10+ years, but because Wizards just recently made it illegal again the Admins there have been insanely anal about even the mere mention of using pdfs. They hand out infractions like free candy over it.