The biggest changes would be relatively vanilla-like pvp and gearing, so you miss out on some of the later twink meta gear like cleft armor kits. However, there's a great deal more gear options than vanilla. My personal favorite is a dagger for horde mages only that actually has spellpower on it, making it replace evocator's in my main build.
Additionally, you can enter AB at any level as well so you could do 19s in a way I haven't seen elsewhere. Haven't seen it pop, however, so it would take an effort from twinks to make it happen. PvP isn't cross-faction, but PvE is and this effectively makes the server pop feel much larger since you can interact with twice the pop of a similarly-sized server. This makes gearing a lot easier if you don't want or have a main to do it.
They've added their own versions of goblins and high elves (basically just blood elves but on Alliance and with different racials) and goblins in particular are very good in wsg. There's a few new abilities, 12 new enchantments so far, 14 if you include armor kits and belt buckles, and existing armor kits are modified.
The economy is monitored by the staff and has been positively affected by changes, particularly changes to some alchemy recipes. There's also several good ways for a 19 to make money. Fishing, like always, and some low-level herbs, but also racing at the shimmering flats can net you raid consumes to sell on the AH as well as raw vendor gold of a decent level if you just want to sell things.