I'm pretty familiar with the following forums (and I just looked at OpenPhotographyForums at it's fairly easy to categorize)
TPF - lots of activity, lots of new, often young photographers, good number of experienced photographers who don't past their own pictures a lot but do participate, critique is uneven with some good but fair number of 'attaboys' held in check by more experienced people, pictures posted range from horrible to pretty nice with the average being in ehh category, under-moderated thus snarky and bi-polar behavior sometimes prevails. (like a school cafeteria at lunch when the teachers are absent, no mayhem but food fights)
OpenPhotographyForums - not much activity, much older group, fairly to quite experienced, quite polite, pictures good quality but generally not very exciting. Home page posts in news and articles 2.5 years old. (teachers' lunch room)
NikonCafe - lots of activity with approximate age spread quite like TPF, pictures range from ugh to quite skilled, critiques are heavy into approval and attaboys (truth might be forbidden and lots of cliques), heavily moderated and any conflict is frowned upon and banished. (lunch room at Catholic school with all the nuns watching)
NikonGear - much quieter, much older and much more skillful group, Images range from technically proficient to sublime, not much in the way of C/C, most members are proficient so that this is a show and tell but when another member does critique they clearly know what they are saying. Moderation rare and usually unneeded, members keep things pretty even-tempered. Pay site for technical boards. (lunch in faculty dining room) [I am a mod at Nikongear and post there]
1x.com - very active, much less of a community feel, excellent critique and superb quality. If one isn't a paid member, postings must be approved for quality. As good quality as there is on the web.