#1. security.  When you make a bitcoin payment; that one payment address then forever connects you to your wallet thus allows the receiving person to see all your previous receives/sends, and, all your future receives/sends.  Any payment you received, or spent, then crushes your anonymity.  You are forever outed; fully exposed.  Really, it's no different than everyone in the world watching your bank account; seeing all the payments coming in and going out.  Bottom line is you can never send or receive a payment to anyone in the real world.  Your bitcoins/activity will forever be linked to you.  Chainalysis perfectly lays out the problem.

#2. non-fungable.  We like to think of bitcoins as being fully fungable.  But the reality is that any bitcoin wallet/coin can be black-listed or otherwise "tainted".  Your wallet could be black listed for any reason, or by mistake with no recourse.  Black-listing means that anybody can create a list containing your wallet address, and then any bitcoins in that address, OR, any bitcoin that ever TOUCHED that address can forever more be treated differently.  If YOU are found to have ever "touched" a black-listed coin, or coin from a black-listed wallet, you could then be targeted, hunted down, your real world accounts/property froze/taken; you arrested, who knows what.  Regardless, not all bitcoins are perceived to be "equal".  And, EVERY bitcoin has a history trail (moving from wallet to wallet) that anybody can easily follow see #1 above.

Bitcoin can never really be great unless every coin is truly fungable.  AND, to be fungable then coins must not be trackable.  Yes, some "alt coins" try to deal with the above.  But ALL mainstream cyrpto currencies especially BITCOIN need to start taking these issues seriously.