<ul><li />Annoying Security Center treating users like absolute idiots.
<li />Installing it reverts all carefully tweaked customisations (like disabled firewall or other system services) to their defaults because MS think's they're safe for the average dummy user.
<li />Crippled TCP/IP stack and operating mode, making more advanced networking tasks impossible due to slow speeds, or just plain impossible.
<li />Needs an extra 1 GB disk space on top of your current installation. Bit of a bloaty waste if I'm gonna go disable all the new pointless stuff it's added for the benefits of the not so tech-savvy users but are useless to me.
<li />I'm not going to risk having endless network connectivity problems I've heard others get. I suppose this is more a risk than a solid fact that makes SP2 bad.</ul>
I'm sure I'll think of more
I wouldn't put anything past SP1a on my machines for now, with the exception of certain critical security patches.