There is nothing more limiting than choosing a great font to work with for the net. While web designers are incorporating far more interesting fonts into webpages the fact is that using a script style font on a web ad the size of your three fingers is going to be an eyesore. period.
Yes, you might in fact see people attempt this, or they might make all the copy ALL CAPS. But ask yourself outside from a design perspective, can you actually read what the product is?
So with that I believe that you should more often than not stick to a sans serif font, fairly bold, not to condensed and upper lower case. I love Trade Gothic, Helvetica, Futura for my sans serif fonts. And if i want to add a bit of spice i will add at times a serif font that can be as simple as Electra, Times, and even Carre Noir. I am a firm believer that incorporating two different fonts into an ad gives it a whole lot of SPICE without using an image as a crutch. Try it out, it can be as simple as making some important words a slight italic, or bold.
More importantly stay away from UPPER CASING your entire copy. We can't control the copy we receive from clients, but we can manage the way it looks on screen. UPPERCASE words should apply to the CTA, a few selected words, maybe the product name, but definitely not the entire body.
With that being said a far more simpler discussion is color. For this you will need to do just a bit of research on where your ad will be seen. The major reason is because you do not want a white background ad, on a white webpage (which is usually the case). Same thing for black background ads on black websites. The ad should blend incredibly well with the page it is on so that the reader never feels like they are being attacked, or worse that the ad disappears into the page. It's never clear cut and dry, but I will say this a thin rule around the ad can never hurt. I would encourage it on most web ads.