All of the above depends on the MUD.

'Search' is commonly used to find hidden objects or exits. But there's no command like that in my MUD. There are a couple 'extended descriptions' in some rooms and on some objects, so if there's a tapestry in the description (but no decorative object in the room) it might still be possible to 'look tapestry'. Outside of the main areas in the cities though, they are few and far between and usually overlooked.

Also no autoquest system at the moment, quests are run by staff members, some announced, some spontaneous. This might change as the game evolves, actually looking at putting in some rudimentary action systems for a newbie school-type area.

Leveling skills and abilities, likewise different for many games. The core component of the code we built off of is stripped of all the old leveling and skill aspects, and we are based off of 2nd ed. AD&D, where levels start out fairly common for the first couple, then get progressively harder and longer to accomplish from there. Skills are similar to proficiency system, with single points to assign. Some are passive and provide a bonus or feature, others the PC masters with practice. One of the skills allows the PC to learn if someone else in the room demonstrates the same skills they know. But a new point is gained only with each new level, and the ability scores assigned at creation do not change without a magic spell or magic item.

All of these questions should be best described by a player or staff member of the particular game, so I would keep an eye out for the ones with a global ooc/newbie channel and helpful people to answer your questions!