The main reason people hate it is because of some of the nastier things it has been known to do... The older variations more so than newer ones have been known for doing everything from installing trojans to messing with files they aren't supposed to. One of the most notorius currently used DRM is Securom.
Securom installs itself without any warning, and the files for it cannot be easily gotten rid of. And software which was unwanted by the consumer and installs itself without warning or easy uninstallability is concerning to many. It has also been known to slow down some computer setups and rarely do other odd things. It is the base software of many DRM methods, such as those below.
A method used commonly now in DRM, especially by EA and other big evil gaming companies is limited installs. The game will only allow the buyer to install the software X amount of times, the details of which vary from software to software. The worst of this sort of system used recently, or rather threatened to be used, was the planned DRM for Spore and Mass Effect (PC). They were planning on putting a 3 install limit on the game, which includes reinstalls done when you do things such as buy yourself a new computer, installing a new OS, ect... the only reason that did not go through was the HUGE protests on the EA forums and just about every other forum and article comment on the web. Once your 3 installs were used up, if you wanted to get any more installs, you have to call EA tech support and argue with them for awhile.
And then there is online checks. Online checks are fairly common in multiplayer games, where on loggin the game checks with the maker's servers to ensure the CD key hasn't been distributed to others or cracked, since the same CD key showing up in 250 locations around the world usually means it is a cracked key. This method is not to bad as long as used only for multiplayer. The problem comes when this online checking is done for single player games. Three that come to mind are the Spore/Mass Effect (again) and my Company of Heroes:OF. As for the first two, along with the above mentioned DRM, EA was planning on making the games check via the internet every day to make sure the key wasn't cracked. If they were not able to connect to the internet, it would give (I think it was) 20 days before it automaticly made the games unplayable until it was able to connect. This would be fine for Spore, which uses online databases in the game, but Mass Effect is entirely single player, and thus requiring an internet connection to play is bad. Company of Heroes: OF I got for Christmas last year (2007). The game requires you to create an online account with them to even play the game. If your internet is not able to connect, you cannot log in to their servers and so cannot even play a single player game. Another BIG worry about this method of DRM is that if the company which made the game goes under, the server would likely go down making the game completely unplayable unless the bankrupt corporations decided out of the goodness of their hearts to patch it out before going down; which needless to say is a mediocre chance at best.
And now you see why DRM is evil! And now you have 'wall of text syndrome.'