To add from what everyone is saying, the only real new system that you missed is strikes. Basically a 10 people single boss battle, like raids but without the filler and pre-events. Very puggable, especially what people call the easy 3 (shiverpeaks pass, voice/claw of the fallen, fraenir of jormag)

If you want to avoid story spoilers, be sure to follow the story until the end of the norn map of IBS before joining them.

EoD added a new mount, skiffs (boat), plus some new systems that you won't find outside of those maps anyway, so don't worry too much about it. But of course you need it to use the elite specs

The last expansion instead gave us new weapons without the traitline and unlocked elite specs' weapons to be used regardless of the spec. To unlock all of that you need to play the expansion until a certain point tho. If you don't have the skyscale in the expansion there is also a new way to get it that should be a bit faster

In general, there wasn't any big shake to things, it's mostly more of the same and some new side things. The only real difference is a bit of powecreep and that now every class can cover any role.

So take your time, play everything in order and all the new things will appear step by step