Hmmm some of the points that I'm thinking of are easy to list...but I don't think I'll find 10...
Heres a few, in order...
1)
Having the right music for the guests. Either just by having a simply massive music collection with you, or for some functions IE weddings, by sending out a "music sheet" for the bride and groom to fill in a month before hand. Mind you, even with music sheets, I've still had weddings where no-one told me that they wanted almost the whole night to be "latin american" - I can do about 2.5hours of L.A (longer with seamless loops

) but after that things get a bit desperate, and I would'nt have know a Merengue from a Salsa back then either. Also, at another gig, having the brides father come up to me whilst I was setting up and him telling me that "they all" wanted Line Dancing...but NOT Country & Western Line Dancing..."Contempory" Line dancing...had me for a while, until I downloaded some track listings...(turned out I had loads of "Contempory" line dancing tunes! - like Atomic Kitten: Whole again

)
2)
A memory of good (& bad) music Here, I'm shying away from calling it "Music Knowledge". I don't think its a requirement of DJ'ing to be able to name all the members of the Shadows, or what the highest chart position of the Venga Boys is... BUT knowing what songs will "go down well" if the song that you're already playing is "going down well" is great.
3)
Reading the Audience They've been up since the crack of Dawn (must get that fixed!), they've had a morning of getting ready in a panic, the boring church service, the even duller photo session, they've passived smoked themselves into kippers during the wedding breakfast, fallen asleep during the 19th page of the "father of the brides" speech about "Its not losing a daughter, its halving the phone bill", they're in the same clothes they were in 12 hours ago, and now, you're asking them to Rock'n'roll ????? Seriously, young audience, old audience, MIXED AGE audience (Ohh I love 'em)... Request slips and requests by text/SMS can help tremendously... also throwing in a few of the slow ballards half way through the night, rather than leaving them all to the end, can give some audiences that "little breather" that they needed inbetween fast/energetic tunes. What era/genre of tunes are being received well, which ones aren't.
4)
Personality inc. humour There are two things that will get this across. What you say, both over the mic, or indeed in person to ANYone that you speak to. I once got several bookings simply from being nice to who I thought were just the venues caretaker and his wife...they were also commitee members of several local members clubs etc that used that hall...Im still getting work from that...hopefully because of my show. While on the mic, make a few "caring" comments especially with the older crowds..."How's everyone finding the volume?", warning them in advance of the buffet being cleared away, pointing out to everyone at the back, about the couple snogging so hard on the dance floor, that oxygen is gonna be needed...
5)
Equipment Someone who I learn't a great deal from, told me that a "good" DJ could put on a great show, with just 4 candles (or was that Fork handles?) and an Amstrad. I think that todays punters expect more than that, although I disagree with some local (to me) advertising that I've seen from a Disco thats probably still in the payment terms of their first HP agreement, advertising "If you're about to book a Mobile DJ that doesnt use Martin Lighting, don't bother". OK, I myself have got about £4K worth of audio gear prior to the amps/speakers, some of which definately helps make the show more flowing...and less of a compromise in some respects, and certainly more professional, but I don't advertise against DJs/shows that just use an old Citronic Hawaii console, still in its purple cardboard carry case. (

ahh Idea for new post). Also, knowing how to improvise a "get you to the end of the night" solution, should things go wrong.
6)
Appearance I guess that this should have been a bit higher up, but not much...I have once or twice, forgotton (see point 7) my bowtie, or my shirt buttons, or shoes, or something thats affected my appearence...OK regretable...but certainly not deliberate, and not as dire as forgetting some of the above points...eg: leaving your mics at home would be worse. However, wearing the right attire (not Dunlops) for the occasion is important. Wearing the wrong thing..especially if its a forgotton item can be worked with...I forgot my shoes once...just had my "setting up" trainers. I wore those...walked right out on stage and pointed out my trainers...holding high my roadies shoe...claiming that it was my only remaining shoe that hadnt been lost in the deep mud outside the halls fire exit. ho hum.... remember too, nice neat haircut etc.. (by the way...The tax man wont take a receipt for "white Fillings" as "On-stage presentation"...
7)
A good memory8)
Flexibility Apart from repeat bookings for almost identically attended do's, like Sunday nights at the local Working Mens clubs etc... you've got just a few hours to make an impression...the RIGHT impression. Go that extra mile... heres an example. A dinner dance back in January...during the dinner, it was traditional for the attendees to "toast" or "take wine with" other members...eg: They'd bang a spoon on the table ("order in court" style), Stand and give a brief (sometimes) speech about some witty event that they had been involved with, with another member, or members.... On my part, I'd just fade out (100%) the background music...but people still couldnt hear from one end of the 70ft hall to the other...my solution...I became a radio mic "runner" for the entire duration of the meal...They'd apparently always had this problem...and no other DJ had ever solved it for them. Repeat booking already

PLUS a booking for an entirely different members club, from someone who belongs to both clubs.
9)
Timing Arrive on time (wasn't that an 80's tune by Black Box?

), be set up on time, and just as importantly...finish on time. In smaller venues in particular (village halls etc) theres normally a caretaker "hovering" by your powerpoints at 11:59...in contrast, every loud member of the audience will tell you if you've finished 45 seconds early...

and will always plead for "just one more" even though there are no songs that I know that only last 45 seconds. This whole "finish on time" trick has been made a lot easier for me recently by my new CD deck, which allows you to seamlessly splice out upto 2 sections (of any length) of the original track and store those settings for "instant" recall, everytime you load that CD/track in the future. I can now choose whether to end the night with the original 4 minute version of "New York, New York", or the 3 minute, or the 2.5 minute version etc...I've set up several of the "end of the night" tunes too....just a middle chorus + verse (or two)...snipped out here, an instrumental break trimmed there...Happy Caretakers, Function managers and above all of course, guests.
10)
A good memory did I already say that?