While I have never done a survey I can tell you what has made me, and a number of others move on.
Try having a certain black belt that goes to Japan for 6 months, then when he comes back he acts like a cocky jerk to beginners and intermediates alike -- but has a completely different face when sensei or the other black belts are around. And best of you PAY for this fantastic treatment!
There are two critical factors that I have seen in dojo's that have made me (and people I know) move on:
(1) The dojo-cho (head sensei) does not spend enough time mingling to see these little events happen, and because of this the sensei is perceived to be unapproachable and no one feels comfortable going to their sensei and saying, "bobby's being mean to me" cause they feel like a child tattling. And in total honesty, as much as people who study Budo are rarely petty -- we only have so much time in our lives (with jobs and families) to do "something" and if that something isn't good/fun/enjoyable/fulfilling, and then on top of it all we are paying for it (its not that cheap either you know), we *sigh* and wander off.
(2) And while some people are extremely dedicated and willing to put forth a life-long effort, others just want to learn something new and enjoy the experience (much like General Arts in University -- they want to broaden their experiences, but they don't intend to make a career out of it).
Now the reason why I personally have moved on has always been related to number (1), since I am extremely dedicated to Aikido and Budo in general, but I have known many beginners that leave because of number (2) -- many!
How do you fix number (1)?
It's almost all perception. If sensei makes a "visible" effort to make contact with his students frequently, he will gain their trust and confidence. Also dojo's should setup up an explicit way of dealing with issues that protects the student from feeling awkward. It will facilitate better communication and resolve issues before the get started, and serve as a deterrent to the jerks as they will be exposed much more frequently -- and then THEY will be the ones feeling awkward.
(2) Is tougher, but it doesn't mean you need to compromise your art (and start offering Aiki-dance-aerobics and such).
Like you said newbies want to feel like they are progressing, and they want to see more of what Aikido really is.
(a) Try having sensei do a 5-minute demonstration, or randori, or aiki-ken, or ninan-dori (something cool) at the begging of every class -- no talking just demonstrating. I guarantee you newcomers will "want" to get into it. The first Aikido class I saw, sensei saw a bunch of us come in and sit on the bench, and "decided" to demonstrate koshi-nage. When he touched the light on the ceiling with ukes' feet a few times, he had us riveted and he knew it.
The other part is "shake it up a bit". Some days should be kata (regular technique practice), some days should be randori (grab-only randori allows new comers to feel comfortable in that chaos -- they will learn to have fun with it), some days should be single-technique shugyo (total emersion into one technique to really figure it out). People will see progress faster, and keep interested.
Wow, sorry for such a long post, but one last thing about partnering a newbie with a "more" senior student. Although the apprenticeship is the best way to learn any art form, you may be de-motivating your regular students that way cause they will be "forced" to work with someone of lesser ability and feel like their not progressing themselves. I think you should have someone mentor them (buddy-up) for the first month only to get them oriented, and then after that they should just get into the "everybody practices with everybody" mode, which gives everyone a range of exposure.
These are just my opinions, and having practiced in your dojo I KNOW it's not your sensei's teaching ability that is lacking -- he is one of the best I have come across yet (to date).