As a former CFI myself, with 2000 TT and 1500 dual given as a CFI when I was hired by a 121 airline 11 years, I can tell you that excellent basic flying skills, especially instrument skills, are probably the most important aspects to becoming a successful airline pilot. The next most important attribute is the ability to work effectively within a team environment.
That said, however, a person who comes into an airline with previous 121 experience is initially (maybe for the first year to year and a half) a much better first officer for an airline and his captains than someone who has had no 121 experience, especially one with no high performance time. There are a couple of major reasons for this.
First, an airline's Ops Specs govern how the airline shall be operated. While it may be true that each individual airline may have slightly different procedures for accomplishing tasks, each airline's Ops Specs are almost identical. The reason for this is that the Feds are the ones who produce a generic Ops Spec. This Ops Spec is then modified slightly to suit the individual airline. For most intents and purposes though, the Ops Specs are basically identical. Therefore a pilot with previous 121 experience has had an experience that will transfer very readily to a new airline. Additionally, he has had experience in the environment dealing with weather. He has probably flown in weather that would ground a light plane, which has given him valuable insight that he brings to his new airline. He has experience with radar - something most GA aircraft do not have and a piece of equipment that takes a while to become proficient at using and interpreting.
Secondly, aside from basic airmanship, airline flying is very different from other types of flying, especially GA. The aircraft are at a performance level that is an order of magnitude higher than most any GA aircraft, aside from corporate jets. Climb at 250 and 2000 to 4000 feet per minute to 10,000', then transition to 300 or so Knots indicated on up to the flight levels. Things happen fast. Today’s RJ's are all equipped with state of the art glass cockpits and FMS's that are complex pieces of equipment. These take a while to master.
So while the GA pilot with no high performance time will eventually catch up to his more experienced classmate with previous 121 time, initially he will have his hands full. He will probably have his hands full for at least six months before he starts to become comfortable with being an airline pilot in a jet. In the mean time the captain will need to keep a more cautious eye on his new FO.
Having flown with FO's with many different experience levels, I can tell you that the pilots who come into the cockpit with previous 121 experience generally have a much easier time adapting to their new airline than those without 121 experience, especially those who have only GA time. Almost all figure it out eventually, though, and become EXCELLENT airline pilots.
