The number of new users registered this month is at it's highest rate since March 2014, when it was at a all time high. January 2015 saw more new user registrations then the preceding two months combined (although Nov and Dec 2014 were very low in terms of new user registrations - I am not sure if Nuked users are removed from new user registrations or not). January 2015 saw new user registrations of almost double of any money throughout the second half of last year.
The number of posts are down substantially however this is likely because of the closing of the Prime Dice signature campaign along with better anti-spam measures taken by BadBear that has weeded out spam substantially.
The number of posts however is still greatly above 2013 levels which is probably a better baseline to go off of as 2014 saw a lot of spam, and a lot of users were attracted to both bitcoin and the forum in late 2013/early-mid 2014 because of the massive price rally we saw in late 2013. The fact that the price of bitcoin has fallen substantially likely means that less people are interested in it (a lot of people who were attracted to it, were likely hoping to "get rick quickly" and when that did not pan out, lost interest - this is evidenced by how many manic posts there were in mid 2014 about the price going to ridiculous levels [outside the speculation/economics sections]).
-currency exchange section is slower then ever.
The currency exchange section is only one part of the marketplace, which is only a part of the overall forum. There are becoming more and more ways that people can easily and cheaply trade bitcoin via services like circle and coinbase which are generally safer then trading in a peer to peer fashion with escrow. Additionally I think there is some level of fear about having to comply with AML/KYC laws that are difficult to understand (these fears are probably mostly irrational, however they probably are there).
-posts are becoming more and more spam-like
Spam is up somewhat, however it really is not that bad now. Spammers will eventually get weeded out and will eventually go away. I would say that some of the spam is attributed to legitimately new users who may not understand 100% what is being discussed - they will likely eventually learn and their posts will appear more intelligent (less spammy) over time.
-people are selling their accounts left and right
There is no reason not to embrace the free market - that is what bitcoin is all about isn't it?
-the overall mood just doesn't seem positive anymore
That is very subjective. However there are a lot of scammers who are unhappy that it has become more difficult to scam for a number of reasons. Additionally I would say that people are generally not happy with the low price of bitcoin which may spill over into other sections.
-other stuff that has zero to do with the forum-
Congrats