The percentage of players who did Sunwell was not particularly high… BUT that is not a good indicator of the percentage of players likely to do Icecrown Citadel. Raiding, and getting to the point of being “ready” to raid are much more accessible in WotLK than in prior expansions. Learn the best rotations for your class, do a few heroics, get some crafted and quest gear, read up on the encounters, you’re ready for Naxx. There are plenty of guilds doing Naxx, and likely will continue to be for some time; the learning curve on some encounters is steep (check out the “Safety Dance” portion of the Heigan fight), but it isn’t as hard to get geared up and “ready” for raids as it was pre-WotLK.
In terms of time investment, even casual guilds are clearing most of Naxx in a single night now. Learning takes a little longer; figure 4-6 hours a couple nights per week for your current raid content, depending on your guild. As your team gets better and learns the encounters, it takes less time. Most guilds will have a raid schedule and expect raiders to be online and available to raid specific nights; more casual guilds won’t have strict requirements, but they won’t progress very quickly, either. If you’re really interested in raiding, figure out if you want to raid casually or “hardcore” and research guilds on your server. If you just want to get your feet wet, you could try pickup raids, just be aware they may be painful experiences just like instance pickups. If you PvP at all, when you hit 80 try Wintergrasp and join an Archavon pickup after your faction wins the battle - it’s a great entry-level raid, very easy boss, with some nice rewards, and will help give you an idea of what it’s like to work with a larger group.
As for pre-WotLK raids… eh, some people do them for laughs or achievements, but don’t expect to find a group of people running Molten Core at level 60 or Karazhan at level 70 for experience or loot.