EDIT: i should emphasize this is only my experience as a startup hiring manager. maybe YC attracts nothing but geniuses, i dunno. i'm just a guy on the internet.
since yc is obviously not going to talk shit on their own applicants, i'm going to go out on a limb here and guess that it's just like every other job application process when six figures (or more) of money is on the line. big paychecks attract plenty of, shall we say, 'aspirational' applicants.
1. apps where spelling and grammar is clearly not a priority (sure, i'll give you 5 figures a month to spell shit wrong)
2. the wrong kind of company will apply, i.e. a restaurant
3. bullshit artists gonna bullshit, even when they stink
4. people simply not smart enough to do the job. just plain dumb is a real thing, folks.
5. people with too little real world experience
6. really stupid ideas (who's to judge? well, the people with the money get to judge, that's sort of how applications work)
the fact that you can use the term 'anonymized data' in a sentence that actually makes sense is going to put you in the top 10% automatically - yes, the bar is that low when you're dealing with the general public. the trick is getting into the top 1% though. it's a power law distribution of difficulty.
every job advertisement attracts applications like these, and yc is basically just a really, really fancy and structured super-duper college/job application process combined, but for crazed entrepreneurs (like me). you are going to get a lot, maybe even mostly people on the left hand side of the normal distribution of iq, talent, and work ethic/experience. the easy thing is the dipshits and fakers are easy to spot-and-drop almost immediately. i can basically glance at a resume and disqualify it in 10-15 seconds if it's not good enough. you just can't bullshit someone far smarter, more experienced, and better at bullshitting than you. CAVEAT: there are extremely smart and experienced bullshitters out there though -- that's the real danger area. that's what you're really worried about.
basically the only thing you can count on is that they want your money!!