A T-tail has structural and aerodynamic design consequences. The structural considerations are of course the increased weight of the vertical tail due to now having to support the forces and moments on the horizontal tail, including strengthening for flutter. The vertical tail can be shorter due to the end plate effect of the horizontal tail, and the moment arm to the CoG is longer - however for most higher subsonic speed aircraft these effects merely reduce the weight penalty.
The T-tail stays out of ground effect for longer than the main wing. Upon approaching the ground, the increase in wing lift causes an auto-flare: the aircraft lands itself. From the wikipedia page of the Handley Page Victor:
One unusual flight characteristic of the early Victor was its self-landing capability; once lined up with the runway, the aircraft would naturally flare as the wing entered into ground effect while the tail continued to sink, giving a cushioned landing without any command or intervention by the pilot.
The aerodynamic consequences of a T-tail have most to do with stability and control in stall and post-stall behaviour, and can be grave. The Fokker 28 and F100 had stick pushers that acted upon detecting a high angle of attack, making it pretty much impossible to keep the columns at aft position. The reason for this is the reversal of the $C_M$ - $\alpha$ slope of T-tails, as depicted below.
- Graph A is for a tail height of 2 * MAC
- Graph B for 1 * MAC
- Graph C for same height as MAC
The aeroplane is aerodynamically stable when the $C_M$ - $\alpha$ slope is negative, such as in cases B and C. For configuration A, the slope becomes positive after the stall point, meaning that the nose wants to increase upwards after reaching the stall - not a good situation.
The stall speed must be demonstrated during certification, and safe recovery from a stall is a requirement. A stick pusher prevents the aeroplane from entering the deep stall area.