When addressing students, can I use "dear students" instead of "ladies and gentlemen"? Likewise, can I address my colleagues as "dear colleagues"? If not, what is a valid alternative?
I think the decision to use "students" or "colleagues" after "dear" depends on the relative position of the addresser and the addressee(s). For example, as a professor, speaking from a position of authority over the students, it wouldn't be considered too gauche to use "dear students" while a student addressing his follow students might use "dear colleagues." It is worth noting that the use of "colleagues" among students has a somewhat formal feel to it--something to take into consideration when setting the tone of the message.
The question seems to be broad and it typically depends on an individual's relation with the group that s/he is addressing. Also, are you talking about addressing them in an email or in speech from a soapbox?
To many, the word dear is a bit intimate and connotes a personal relationship (BBC). In emails, dear is less frequently seen (I have seen mails quite straightforward, starting with 'Hello' or a bit informal 'Hi').
Having said this, putting 'dear' makes it a bit more personal. If you are addressing them in your speech, there are many options considering your kinship with students or whosoever they are.
The transformation, I think, is by dint of immense use of emails those are more like texts than letters. I'd personally prefer addressing the crowd using 'Hello' if I'm speaking for the first time. If the communication and relationship has been established, using Dear is okay. But again, as I said, this is what I understand and practice. Let others come up with their views.
A side note but worth mentioning: This question reminds me the lesson of 'Swami Vivekananda' being taught in my school days. In his speech in America, he addressed the audience with 'Sisters and brothers of America' and at these words, Vivekananda received a two-minute standing ovation from the crowd of seven thousand!
Good read about the usage of the word dear is here on CNN.