Of course, there are stories of this thing completely changing the game, but in general, there is usually enough enchantment removal floating around that most of the time this will probably do nothing (expect possibly draw out that enchantment removal, but you dont usually want to approach the game like that, especially in multiplayer)
While other cards do similar things, like Nantuko Mentor, Xenagos also gives the creature haste
Whatever its doing, the state it's in, whatever's attached to itall of it stays the same
It doesnt matter if that land has other card types like Dryad Arbor or Ancient Den
Its such a pain to play against, too, as it does the things you dont want it to at the worst possible times
Smothering Tithe This scales with multiple opponents and if someone at your table casts this, they need to be mercd immediately