I always felt the biggest mistake was the fact that they moved away from the old familiar models. In most cases, I slag the sequel off for being too much like the original ,but in this case, bugs aside, the main problem with Frontier for me was that they changed the entire universe from the Physics up, the only thing it really had to do with the Elite I knew and loved was the name. If Frontier had had either limited or at least easier to deal with Physics, the same Engine otherwise, and a more familiar feel to the ships, I think I'd have enjoyed it a lot more.
Combat, I think, was the most dissapointing part of Frontier.