Some of the comments here demonstrate a lack of understanding on both sides of the R/T. Whilst the slot system and -5/+10 margin is for the management of the local ATC environment we as Pilots have been using this as taking up the daily delays of airport operations. So the obvious question is whether this present system is fit for purpose. It is a reality that on any given day that small delays in the very complex operation of preparing an aircraft for dispatch have to be accommodated by the system, equally if an aircraft is ready to depart earlier than schedule then it would seem ludicrous to keep it at the gate whilst the passengers and crew count down the clock. As is often the case when systems become more and more automated and computerized they become less flexible. We as an industry should not allow this to continue. There needs to be more operational awareness brought into these projects at the design stage to arrest this ever increasing problem of technology creating life less flexible and user friendly to us the users.
On another point regarding planned and actual flight levels. Obviously an aircraft operating FL is in part governed by it's planned take off weight. For a number of reasons, not least commercial, this planned FL may not be possible due to higher take off weights or a higher, more efficient, comfortable and faster FL may be possible due to lighter loads. In either case the system has to be able to deal with these situations and controllers should have some understanding of the reasons behind pilots asking for FLs other than those filed. Commercial awareness needs to be higher up all of our thought processes.