The topics are, unsurprisingly, very similar. Tradition holds that the church at Thessalonica was founded during Paul's second missionary journey. He clearly spent some time there personally teaching them (2 Thes 2:5, 3:7). The letters may have been written in 50 and 51 AD.
The first letter offers encouragement and hope in the face of persecution (including tantalizing information about Jesus' return for the Church).
For the second letter, it is clear that the persecution is still ongoing (chapter 1). On top of this, there seems to be some sort of false teaching going on (2:1-3a). Apparently, someone claiming inspiration of the Holy Spirit, and falsely claiming the support of Paul has told them that they are living in the last days (the Great Tribulation, although Paul does not use those words).
What is Paul's response? That the Great Tribulation is only metaphorical, that things will get progressively better as the Church renews the world?
No. He tells them that there will be a great rebellion (probably spiritual, but possibly physical or both), that a "man of perdition" (or destruction) will rise up, and that all the unsaved will be deceived.
Paul here tantalizes us again:
"Do you not remember that I told you these things when I was still with you? And now you know what holds back, for him to be revealed in his own time. For the mystery of lawlessness is already working, only he is now holding back until it comes out of the midst. And then the lawless one will be revealed..." (2:5-8a)Um, no Paul I don't remember, I wasn't there!
The King James has "he who now lets will let", which shows the importance of updating translations. "Let", to us, means "allow". But at the time of the KJV, it meant "deny" (or block). Thus, the Modern King James has fixed this to be "he is now holding back". The second "will let" was actually added - it is not in the Greek. But the idea is confirmed by the later "until he is taken out of the way (or midst)".
Dispensationalists hold that this is the Church being taken away, reducing the mediating effects of the Holy Spirit in the world. It makes a lot of sense, but it is not an open and close case from just this passage.