New multiboxer just starting out!

Just wanted to thank all you guys for the help you've put up. I've been reading here for a while before I finally took the plunge and bought a few more copies of wow to start a team with.

Right now I'm in the low 20's, but hope to cruise along and be 70 before wotlk comes out!

I had already leveled up a few 70's and a couple other mixed level characters, so I've been through all the content numerous times, and this leaves me some options for tanking/healing when I get everything leveled to 70 (I've got a 70 paladin, warrior and druid already).

Right now I'm leveling 3 warlocks and a priest up until I catch up to a 63 warlock I already have, so I'll have a 4 warlock/1 priest team once I get to BC content.

My question is, are there any noticeable slumps in leveling up? I'm thinking the 30's will be rough until I get to SM content. And maybe from 40-50 will be tough? I plan on enjoying the 50-ish level instances for nostalgia (Dire Maul, LBRS, BRD, scholo) before heading to BC.

Since I'm a regular raider 4 nights a week it's tough to find time to also play these guys, but they are quite a bit of a change-up from raiding and I think I'll really enjoy it once I get them geared.

I'm currently leveling on Illidan (US) Horde side - guild name is March of Souls (4 warlocks 1 priest).