The last episode of Enterprise sucked...and I'm actually an Enterprise fan!
Just as it was about to go down the tubes with a whimper they saved it...in the fade to the closing credits.
No spoilers for those who have yet to see that episode...but let me say this:
There are plenty of letters left in the alphabet.

CraigL, in "Trials and Tribblations" (DS9) they touch on why the TOS era Klingons have no head ridges...but in Enterprise they actually explain both the loss and the eventual return. Worf is not very forthcoming about the reasons though.
Funny actually, because an alternate explanation had been part of canon...it was violated by DS9, reinforced by Enterprise...and it's Enterrpise that gets all the bad fan-press for canon violation.

The true failing of Enterprise is in the writing...they dumbed the series down and concentrated more on an "action" format than on the tried and true morallity play that classic Trek encompassed. Even the change of the theme between seasons 1 and 2 seemed aimed at a younger audience...instead of simply aiming at an audience that wants to think.