I've had both Dish Network and DirectV. Friends have Comcast.
1. Reliability -- Comcast has far more outages and problems than either satellite provider.
2. Weather related outages/problems -- Both satellite providers deny this is much of an issue. Totally untrue. Thunderstorms and snowstorms take the signal offline for sometimes as long as 14 or 15 hours. Snow needs to be cleaned off the dish, so location of the dish can be an issue for some. Brush it off early before ice forms. Wind can blow the dish off a bit and compromise or take the signal off. Two people, one outside twisting the dish, horizontally in my experience, and a second inside to view the signal strength are needed along with some way of communicating about signal strength.
3. Customer service -- Comcast has been especially slow and awful for those I know with problems. Dish Network reps are rude. DirecTV is generally okay, but check your first bill, none of the "special offer" prices I was to receive were there. A call to them quickly rectified this. I have heard this from others.
4. Technical assistance -- As above, though with problems getting above the very minor level, good luck. Level 1 support for the satellites consists of folks reading to you from a manual without affect or understanding of your situation or what they are reading. Getting to the next levels is sometimes takes a bit of persuading.
When I migrated to the HD dish, the tech first sent out would not install it because signals from only 4 of the 5 satellites could be received. I asked what channels were on the satellite I could not receive because there was some chance that I would be willing to forgo these. It was as if I were speaking a foreign language. His supervisor was to call me to discuss this. No call despite repeated calls to the contract installation company. Repeated calls to DirecTV to eventually learn that my order had been canceled because the install had not taken place within 5 days. Before I re-ordered, I tried to take this up with DirecTV. Again the foreign language feeling. One rep repeatedly refused to let me go to the next level and wondered why I was asking such a question. Persisting, I finally got a tech supervisor who was the first person who had any understanding of this seemingly simple issue. I was told that the information I was seeking was not really available to any of them, but working with what they knew and my preferences suspected that it would be all right, which it turned out to be. When the same install person showed up, he ended up putting in a very long time choosing a spot and aiming the dish, dug a hole, installed the pole provided by me in concrete provided by him, and made a very long run of wire, so that I should later decide how to route it and bury it, all at no extra charge. He then did a rudimentary but good programming of the remote for the TV and their box. I gave him a substantial tip and a beer.
5. HDTV -- I don't get all of the channels and do not get any discount for this, but this was part of the deal by accepting the compromised install at my location. Shotime and major network HD is visually much better than HBO, though HBO is better than any non-HD.
6. If you decide to leave one of the satellites, esp. DishNetwork, the fun really begins. You are told that you can never get any special offers from them and this applies to anyone at that address, in perpetuity (their words, not mine here), so that your wife, roommate, or dog cannot sign up to get better receivers, new services, etc. with low prices, even after years and years as a regularly paying subscriber. After a while, the "come back" offers start, but they are distinctly non-generous compared to what new subscribers get. So....
7. Upgrading your receiver can get pricey, and the receivers they sometimes first include are not so good. The first receivers I got when I migrated to DirecTV totally blocked out the picture when you were surfing via the channel menus. Fortunately, Circuit City has a blowout table where returned receivers were frequently available for as little as $5, so I upgraded twice with more satisfactory results, while having to extend my contract for one year.
8. Leasing the equipment -- Now,unlike when I first signed up, it seems that you lease the equipment, even if you had to pay for it in the first place. Terminating means that you have to send it back within a specified time or pay a highly inflated price for it.