If asked for the prototypical slot receiver, I am sure many NFL fans would name Wes Welker. Welker has been the best slot weapon in the NFL for close to a decade. Typically you will hear the type of player teams want as somebody who has a big initial burst of quickness in a short area and is elusive in the open field. Size and long distance speed aren't important. Since slot receivers line up behind the line of scrimmage, they are difficult to jam, and they tend to run shorter routes where an initial burst is what creates separation.
Welker has certainly proven to be a top weapon. You might dismiss him as a product of playing with Tom Brady and Peyton Manning. I would counter there is a reason the Patriots and Broncos went out and got Welker to play with their quarterbacks.
The Welker model at slot receiver is not the only way to go in today's NFL, however. Teams are more and more looking to create mismatches in the slot by sticking big receivers and tight ends there. Slot corners tend to be smaller and less physical so having a big target who can overpower the slot corner is another approach.
The Saints are foremost among these teams. Both Marques Colston and Jimmy Graham ran around half of their routes from the slot last year. Anquan Boldin, Larry Fitzgerald, and Brandon Marshall were slot receivers over 40% of the time. The same went for fifteen tight ends.
What does this mean for the Draft? Some people are skeptical of Eric Ebron and Jace Amaro because they do not profile as prototypical in-line tight ends. While there is definitely still a spot for the traditional tight end in today's NFL, the most value a tight end can provide is in creating mismatches in the passing game. This is frequently done from the slot. Position labels can get too rigid. It's not about being a tight end. It's about being a slot mismatch. Kelvin Benjamin is another prospect who potentially comes to mind in this role.
NFL offenses keep becoming more and more creative. Don't get too caught up in rigid attributes for a given position.