First up would be "Welcome to the Hellmouth." Everyone is so young! Buffy is all round cheeked. Darla is oddly ineffective. Angel is all mysterious and reluctant to get involved. The Master looks like he has fruit-punch mouth.
So, post your thoughts or even just link to an entry on your own LJ, and don't worry about spoilers for the rest of the series... the assumption is that we've all seen the whole series (both BtVS and AtS).