For a promotion, having a proven draw is like having an ace in the hole. It could be a large number of factors as to why a talent is a draw for fans, but let's look at some of the biggest.
Mic skill---If they can cut a promo, they will sell. Few things can cover up subpar in ring work better than the ability to talk and sway a crowd. Talents like Hulk Hogan and John Cena have made a living off of selling to a capacity crowd with a limited in ring moveset.
Technical Prowess---The ability to pull off moves in succession and make them look smooth both in transition as well as in execution...it takes a special performer to be able to use that to offset what might be missing in the area of microphone skill. Bret Hart, Dean Malenko, and Chris Benoit were professionals in that area and became notorious in their proficiency.
High flying---Maybe the performer can't talk with any compelling talent or put together a chain wrestling clinic, but they have a laundry list of "holy crap" moments. Jeff Hardy is the first person to come to mind for that. His matches may not be top 10 matches, but some of the moments are. Why? Not many other performers would jump from atop a 12 foot ladder through a table and an opponent.
Ring Psychology---The ability to sell a match in ring. If a performer can make the story in ring look believable and cover any flaws by improvising the circumstances, you have yourself something special. Randy Savage (R.I.P.) and Jake Roberts had this in spades.
Adaptability---The ability to work with ANYONE and have a good match because they can adapt to the differing styles, sizes, and talents of other performers. Shawn Michaels, AJ Styles, and Kurt Angle have made this their realm.
While some performers can hold their own in a match, others must be carried. While others can make the story believable, their opponents can create a match people will remember for a lifetime based off of a 15 second spot. Most performers on the top have more than one talent, hence their position. Guys like Shawn Michaels, Kurt Angle, Ric Flair, The Rock, Steve Austin, and Chris Jericho are in the top of the craft because they have more than 2 of these qualities and that is something that transcends the company they work for and puts them among the elite....the greatest talents ever.