We have been cruising for few years now, and wish we had started sooner. We go on mid sized ships, around 1000 passengers. We have both sailed from Southampton, but there can be the problem of the Bay of Biscay sometimes, although it is not always rough. Fortunately we don't suffer from seasickness. We have also done a couple of fly cruises but then you can't take so much luggage as you are governed by the amount the airline allows, plus the security etc at the airports, there and coming home. Leaving from a UK port, although there is now security, doesn't mean you have to arrive as early as at an airport. A very short cruise of 5 nights or less is normally full of drinkers on stag and hen parties, so I have been told, but not been on one so can't say.
Most of what has been said is what we have found. Formal nights are not as formal as they used to be, although some people still go rather over the top. Tables for two are ok if you don't mind sitting with the person you sit with everyday of the year. However tables for 4 can be tricky if the other couple are not really your type. Six or more is probably better, although 10 can sometimes be a bit much.
I could never have an inside cabin, or just a porthole, so always have at least a picture window, although when we get a good deal, we like a balcony. If you are on a lower deck and to the rear of the ship you can sometimes fine the noise of the engines annoying. We were once told if not sure where to have a cabin, then have one in the middle of the ship, both from front to back and top to bottom.
Someone has asked about mobility scooters. These are allowed on ships, but not normally taken ashore or on trips out. Other types of walking aids, chairs etc are usually taken ashore, but not necessarily on a coach trip. It depends on the country and the type of coach.
A river cruise is a very much calmer experience. We have been on one. A lot less people, the boat moors within walking distance of a town centre usually. However beware of ships with very small cabins. Most are fine with a double, or twin beds. However I believe there are a few where they are more like bunks and folded away to make room during the day.