I haven't commented on here in a while, but I saw a Kayle topic and just felt the need to chime in
1. Never build Nashor's Tooth on her. Ever. Stinger - yes, sometimes. Nashor's - Never. It's just a great big waste of money.
2. On-Hit does very well if you are against a tanky bot lane. Build a Malady, Wit's End, Brutalizer & Frozen Mallet and you'll start doing some pretty awesome surprising damage while still being tanky. It's also a build that you can realistically achieve in most games.
3. Straight AD is definitely the best way to dish out the damage, particularly if you're against squishies. If you're runed/masteried correctly, the downtime on your E will be minimal and not really noticeable, which brings me to...
4. 40% CDR, while nice, isn't necessary. 33% cdr is necessary to keep your E up at all times, but since battles rarely last as long as its uptime, 25-30% CDR is a pretty good number to shoot for. Your ult and E will still be up whenever you need it.
5. If you're bot lane against Malphite - swap. He will win. This is particularly true if you go the On-Hit route, as your attack speed is your most important stat. On this note, anyone that gets Frozen Heart will also give you a hard time (like AD Malz, Urgot, Warwick, sometimes Yorick). They're not unbeatable, but they're tough for obvious reasons.
I hope that helps. Kayle is crazy fun.