Pretty much everything that
@King Greg said.
The way satellite pings and the way UO processes data mix together like oil and water.
When moving you'll take 2 steps. Then freeze up. Your comp sends the info through dish, to space, to server. Sever sends data back to satellite, to your dish and then comp. You'll then unfreeze and move 1-2 more steps. This is probably a second or 2 later. Meanwhile everything else around you is moving smoothly for the most part.
Every single action you do goes through the same process.
Toggle a special move. Interact with an item or crafting menu.
Cast a spell and it takes a second or 2 for it to even register and begin to cast. Then you have to remember that in UO you're not locked onto a single static target and it auto casts. You have to "last target" or even click on whatever it is you're casting on to send that info. Then once again it goes through the same information exchange back and forward.
Your best bet is some sort of tamer template coupled with Spell Weaving and a maxed out casting focus. Gift of Renewal's heal over time works decently to pick up the slack of not being able to spam bandages or mage heals.
Magery without eval for self invisibility and the various other utility spells. Not to mention Swords of Prosperity or some other mage weapon for defense.
But you'll still get wrecked by anything that requires you to be really active and on the ball. You're looking at being passive and not active.
Crafting is semi viable. Tailoring and Blacksmithing anyways. Just expect it to take 2-3 times as long to get anything done.
On top of all that your bandwidth is usually capped with a daily limit and then slows down "to 56k like speeds" It's more like 5k speeds. You'll eat through it stupid fast.
I could go on and on about the downfalls.
Go for wireless and 3g. It's miles ahead of a dish connection. Even at 1 bar.
Not to mention the start up costs are flat out absurd. You're talking about a few hundred just to get it up and going. Oh and then you're contracted to most providers as well.
Just don't do it.
Ask me how I know this.