Some sects have picked up on the 'they will speak in new tongues' to mean that ALL Christians must speak in tongues. Apart from the complete absence of any such statement in the rest of the New Testament (and 1 Corinthians 12:30 stating the opposite), there are three reasons why this passage can be shown not to teach this.
Note that this is exactly the language used in verse 16!
So verse 16 (the need to believe and be baptised) applies to everyone. Verses 17-18 (the signs) apply to Christians as a group.
That it [the promise of signs] was fulfilled is evident from casual references in the epistles of St. Paul e.g. 1 Cor xii 28, Gal iii 5, though the formar passage shows that the sEmeia [signs] did not, even in the apostolic age, attend every believer. (tois pisteusasin [the believers] not tw pisteusanti [the believer]). Their purpose was to be the 'signs' of the Divine mission of the church, not to accredit the indiviual.
[Henry Barclay Swete, 'The Gospel According to St. Mark' (MacMillan, 3rd Edition, 1909, p. 405). Emphasis mine]
This is exactly what happened in Acts: each of the signs listed (except drinking poison) actually is recorded in the book of Acts. See The Text of Mark 16 which shows how Mark 16:9-20 is probably an attempt to fill in the missing end to Mark by using the data in Matthew, Luke, John and Acts.
If one is to say, ALL Christians must speak in tongues, then one must be consistent: ALL must drive out demons, ALL must pick up snakes (safely) and ALL must heal people.
It is not good enough to say 'a true Christian will not be harmed if he or she happens to pick up a snake'. That is not what the text says. The text is clear: 'they WILL pick up snakes'. In fact, the text could easily have said 'If they pick up snakes, they will not be harmed' - because that is what it says about drinking poison. But on the bit about snakes, it is an unconditional statement: 'they WILL up snakes'.
The promises of Mark 16:17-18 relate to Christians as a group. Some (not all) will pick up snakes (as Paul did). Some (not all) will heal people and cast out demons (as at least some of the apostles did). And some (not all) will speak in tongues. This is the meaning of the text, and the evidence both of the book of Acts and the history of the church.