I'm going to try and approach these questions as more of a booker and less of a fan. Now i'm not Takers biggest fan, but there's no deneying his overall popularity
1. the gimmick: that gimmick is just WAAAAYYYY out of place in today's wrestling environment.
What about Hogan's or JBL's Millilon Dollar man?
It's marketing, Taker was a recognisable face back in the day and regular fans know him, it's a good way to re generate interest in fans that may have lost touch
2. his work rate: he is not only prone to injury, but his character does not fit today's wrestling environment
To be fair I can't really think of a stand out bad match that he's had since his return of the dead man, there's alot of people out on that roster that are far worse.
3. his entrance: ooooh, scary stuff! wait, no. TOO MUCH stuff.
See Point 1
4. that he is a habitual babyface: Undertaker as a heel =

i thought it was pretty cool when Undertaker went and beat up Ric Flair's son. Undertaker is getting up there in age. He aint a phenom anymore. mean ass redneck who aint been gettin' enough respect, that was a cool gimmick. but then, just when i'm starting to enjoy that gimmick, Undertaker as a face =

always the victim and constantly getting screwed. who didnt know that Heidenreich would end up costing Taker the match vs. JBL for the title. however, simple logic does dictate that the 7 ft. satan worshiper be a babyface, right??
At what point in this new gimmick has he claimed to be a Satan worshiper? And how can he not be a face right now? Honestly. How would you turn him heel? he's the most over guy on both rosters! Also see point 1
5. his fueds: again, this is due to the phenom 2004 switch. his fueds with Kane, and Booker T were just embarassing to watch. i mean, we know wrestling is not for real, but no need to insult us.
Personally i really enjoyed his feud with Kane, granted his booker Feud was a waste of time, but Book was a quick replacement for Brock. His Orton Feud was awesome, his JBL stuff was decent, not great, but then JBL is 90% gimmick 10% wrestler.