Doctor Who was always in danger of cancellation throughout the final seasons. Colin Baker's stint started it - they made his Doctor too alien and inhuman to begin with, which is why he was the shortest-lived Doctor around (a shame actually, he could be quite good given a decent script to work with). McCoy was IMHO the best Doctor since Tom Baker. Remembrance of the Daleks was an almost perfect story, as was Ghost Light. Unfortunately, they spent way too much time on cameos by popular TV stars, daft and self-referential plotlines and many of the scripts weren't too hot. Even worse, the show got axed just as it was picking up big time - after Ace came in the stories got a tad darker and were better written. Battlefield and The Curse of Fenric were atmospheric and very well put-together.
Even Silver Nemesis (the 25th Anniversary Special), whilst not great, was a fun romp and brought back the Cybermen (my favourite bad guys). The dialogue between McCoy as the Doctor and Sophie Aldred as Ace was top-notch:
"Ace?"
"Yes, Professor?"
"You wouldn't by any chance have disobeyed my strict instructions and secretly prepared any more Nitro-9, would you?"
"No, Professor. I'm a good girl and I always do what I'm told."
"Excellent... [grin] Blow up that starship."
Enterprise's problem seems to me to be that the scripts aren't particularly well-written. Given character interaction like that shown above, the effects don't need to be too hot, and the plot doesn't even need to be particularly creative to make a good show. (After all, as White Wolf games pointed out, the plot of one of the best films ever made was just a great big shark terrorizing a seaside community...)
"That might have been the biggest mistake of my life..."
"It is unlikely. I predict there is scope for even greater mistakes in the future given your obvious talent for them."
Vila and Orac, Blake's Seven