I think there are several ways of looking at this: those who have disappeared soon after the start of their big storyline, those whose big storyline was damaging to their character or those that don't seem to appear unless it is part of their own storyline.
Quick disappearances:
Carol promptly shipped off after her operation
Kat not seen much - Should we not have seen Tommy visiting again and coming to terms with her injuries, and potentially Alfie persuading someone to allow the twins in for say five minutes? Also disappearing again after this week when she comes out of hospital. Favouritism for say Shirley is obvious, who only disappears for such a length of time when the actress has a break.
Patrick
It seems that rather than depict a recovery from a serious medical issue, we're getting Linda's recovery from her rape rammed down our throats instead.
Damaging big storylines:
Alfie - The sheer amount of posts on here about how unbelievable it was that he would set his own place of residence on fire (especially with only small hints of his financial struggle - not seeing him mention look outside of the Square for work, get rejected from a number of jobs, etc) has to put him firmly in this category. Instead we just get the storyline plonked in with a few blink and you'll miss them mentions of the job centre and two rejected loan applications.
Dean - More of a grey area since he has been complexly written and given interactions with many other members of the cast, but it's obvious his character has been sacrificed to give Mick and Linda a storyline that temporarily affects their relationship, but is not damaging to either of their characters.
Part of me wants to put Ian in this category because of how unlikeably he came across after Lucy's death, but a lot of it was a necessary plot device. Jane, Cindy and Bobby all had to be written out; Jane because Laurie Brett was contracted to film the final series of Waterloo Road, Cindy because of Mimi Keene's GCSE's and Bobby because Rory Stroud was leaving the role. Also the Rainie business, but I guess that could be looked at two ways. It did give Ian a real OMG secret that didn't implicate him in his own daughter's murder, but on the other hand, it also looked like a plot device to get Tanya Franks back, who I'd guess that DTC saw had previously played second fiddle to Jo Joyner, despite being (I'd say) the more talented of the two.
Appearing only in their major storylines:
Again, Kat and Alfie, Alfie particularly. He was largely absent during Jessie Wallace's break, and the pair popped up again for the birth of the twins and disappeared within a week, so much so that a baby can need to be resuscitated at birth and then be home in the next episode. Alfie is also being made more unlikeable as a plot device to give Stacey more to do without shoving her straight into a big storyline of her own, it seems.
Lola - Plot device in Peter/Lauren it seemed, plus babysitter so they didn't have to pay for Janet to appear in the wedding episodes as well as Denny and Amy. She would not have been so much of a doormat to disappear when Ben turned up, or rather to be convinced by Billy to do so. Ben said he didn't want to be involved with Lexi and added to the fact that Lola is more than capable of handling herself, protecting Lexi and sussing when something's going on, that should have been more than enough.
I'd also put Les and Pam in this category, as Les particularly hasn't done much other than his job and involvement in the Nick's fake death storyline.
Denise, potentially - Shabnam, although probably underutilised since her return, has appeared much more. Denise hasn't done much not connected to The Beales or Patrick.
Oh and Masood but he really doesn't fit into any category.