True Pre-mine - Mining began before it was announced. I think this is ok to a certain extent as long as it goes to the developers and they use it to pay for server costs, giveaways, paying scripters, and maybe a little for them (like shares in an IPO). As long as this doesn't amount to too much of a pre-mine and they were up front about it, then I wouldn't be against it.

Insta-Mine - Friends of Dev's were sitting waiting to press the button as soon as it was announced thus getting super easy blocks before people have the chance to get their miners prepared. This has probably been done by every single coin in existence. It's not super ethical and I think this pisses people off. Setting the difficulty higher upon release would solve some of this issue (New Devs please take note).

Insta-Mine with Large blocks - This is the one where the first blocks are set to pay out several hundred coins then drop as the "days" go on. This is probably the worst of all especially if the Dev's friends are ready to mine the instant it is announced and even worse when the difficulty is so low that after 20 minutes the first 3 days of blocks are already eaten up my these miners. This is the only one that actually upsets me because it is set up to look like it is fair and great for early adopters but in reality it is the most unfair way to launch a coin.

It's a Pre-Mine because I didn't know about it - This is probably the real reason people call a coin "pre-mined" or "scam". It's because they are butthurt that they weren't online when it was announced, the difficulty went up to high before they could get in on it and now the want to make the coin look bad so that they can get people to lose faith in it and therefore lower the hash rate (or prices). Or, they prefer another coin and think that by bashing another coin their coin will look better. This actually has the opposite affect bringing more awareness to the coin.