I was pretty sure WoW used Warden...
In any case, botting in WoW is definitely quite a lot easier than botting in D3. Farming in WoW is basically the act of performing a very easy and boring task over and over. The task is boring to prevent people from doing it more than they really need to, and pay gold to buy items from those who farmed them. Being a good player with good gear may help your farming speed, but not too much. Farming in D3, on the other hand, is doing the most challenging content you can with as much MF/GF as you can as effectively as you can without dying. Being a good player with good gear will have a very significant effect on your ability to farm in D3. Bots won't be farming end-game items in D3. At most they'll be farming gold, upgrade-able crafting mats and low quality gems.
That said, no matter how difficult botting in D3 is, it will still be a problem if botters can get away with it. Not as big of a problem as it is in WoW, because they won't be able to farm nearly as much (compared to what they can in WoW when you normalized it relative to what a normal player would farm), but still potentially enough to greatly increase the availability of gold (and maybe commodities?) compared to what it should be. Killing stuff in normal/nightmare difficulty may suck for farming, but doing it 24/7 might still be much better than farming hell/inferno for 1 hour a day by a normal player.
In the end, how much bots will ruin the game will depend on how hard it is to make efficient bots (aka game difficulty) and how often they get banned. If botters can't make 60$ before they get their account banned, they'll simply stop botting altogether. I guess we'll have to wait and see how good Blizzard will be at banning the bots.
Keep in mind that in WoW I'm pretty sure the botting farms and the likes get their accounts banned on a regular basis. There's a reason they try to rent accounts from people who no longer play and "guarantee" that they won't get banned (offering an alternative account in case of a ban), instead of buying accounts or leveling new characters on newly-bought copies. They need the bot to make more money than what they pay for the game, and if the bot gets banned too quickly and they invested too much in the account, it simply isn't worth it.