Generally a four man premade group will contain at least one ECM mech(usually two), they will contain highly speced mechs(all the upgrades, etc...) and they will communicate easily(duh). Combine this with the synch drops(don't care if you're wanting to play against your clanmates... harden up and form an 8 man team), the high chance that at least one PUGer will dc/afk/suicide rush, the fact that ALL ECM counters require teamwork and you have a high chance of ending up on the wrong end of a backside kicking.
All in all this makes for a terrible, terrible new player experience... having played PUGs constantly I can say without a doubt the four man groups are the most negative element in the game. I LOVE mechwarrior but playing as a PUG just sucks.
EDIT: So, it seems the PUG vs. Premade Troll Squads got into this thread and started wailing and whining and grinding digital bones to make their bread... all of theses actions miss the point. The point is the new player experience. I'm not a new player. I could join a premade and stomp around killing PUG players at will if I wished... so for the aforementioned troll squads... issue is new player experience, not how awesome you are(also no Mechwarrior untill you finish your homework).
EDIT: If you can wade through the nonsense there are some solid observations and points made throughout this thread... don't look under the bridge though.
Edited by Sam Slade, 01 February 2013 - 06:49 AM.