Not sure about intelligent and informed, but here's my thoughts @buzzheath ...
"Although incredibly oppressive, it wasn't exactly an Islamist regime under al-Assad, was it?" Assad's regime was oppressive and cruel, characterised by human rights abuses. Assad used chemical weapons against his own people, his regime tortured children... Something doesn't have to be Islamist to be awful. In his efforts to hold on to power, Assad catalysed a civil war that has torn Syria apart.
"What is Baathism?"
Ba'athism arose from, I suppose the simplest description would be, an Arab nationalist ideology, combining ideas of socialism with a one-party state. The main examples of how this manifested have been Saddam Hussain and Assad, both of whom used authoritarian and frankly dictatorial approaches to holding on to power, so the practical examples are quite far removed from much of the original theory.
"Why have Russia and Iran supported Syria?"
They supported Syria because it was strategically useful to do so - once it was less useful, their support decreased substantially, as we saw. Syria was a useful link in Iran's 'Axis of Resistance', which Iran viewed/views as important for its geopolitical survival (Iran sees itself as being victimised and under threat, and in a fight for its own existence). That network has been crippled recently and that changed some of the equations going on behind the scenes.
"The coalition of groups that toppled the government - are these Islamist?"
The coalition of groups fighting against Assad's regime has been diverse, changed over time, and involves groups of very different political viewpoints. Groups have splintered, re-positioned themselves, fought each other... There are groups that have their origins in some very unsavoury connections (eg Al Qaeda, IS) and which are currently classed as terrorist organisations in the UK (currently subject to review). The push to oust Assad has had Western backing, and the main groups involved have done a lot of re-branding of themselves recently, so whether there are still major players who'll push for Islamism remains to be seen.
"Are they actually pro-democracy? What does it mean for the future of Syria?"
The main groups seem to be trying to assure the international community that they don't want to fight their neighbours, or oppress minorities, but whether that's sincere or not will take time to see. An interim government is being put in place, and a lot of questions can only be answered with "we'll need to wait and see" at this stage. The future of Syria is very uncertain at this time - lots of different groups and regional powers will have their own competing agendas.
"Syrians seem pretty elated (based on photos in the media, etc) - why is this?"
Assad is gone. There is uncertainty over what happens next, but there's now hope where it felt like there was none, and a sense of long-awaited justice in his fall from power. People who have spent years in exile from their homes may now have a future where they can see home again. People whose loved ones were in prisons may be reunited with them, or at least find out if their loved ones are dead. People whose loved ones were brutally killed and had their bodies disrespected may now be able to re-bury them with dignity or at least have a proper memorial for them. The past few days have been an emotional whirlwind for a lot of people, and in amongst the celebrations for hope for a free Syria, there is also a lot of trauma being processed and a lot of healing starting. People don't know what will happen next, but right now, Assad is gone.