I've never really allowed multiclassing...I view it as an excuse make ridiculously broke characters. However, my players always wanted multiclassing in our games (half of them are basically munchkins that just love making the strongest characters possible).
Well, you know your players...and if you think this is a likely possibility (munchkin characters), then you were wise in nixing that option.
After so long, I was thinking of just giving them a Christmas present of allowing multiclassing.
Is it a good or a bad idea? What should I be on the lookout for? What should I be careful about? Tips?
Give them one "last chance" to decide if they want MC or not. Make sure to say "Are you sure
? As in absolutely
sure? I don't want to hear and complaining later on about it...so think about your decision carefully".
Then, if they still say yes, so be it. Let them make MC characters. Make sure *you* also make
-for-tat NPC opposition as well. If they come up with some wacked out uber-DPR melee character...you come up with a counter NPC for said PC. You can be nice about it, and just make it an even fight...or, if the player is being a complete dolt about it (re: taunting other PC's or NPC's, or laughing at the DM about how awesome his PC is and how there is no way for the DM to 'win' against such a perfect build, etc), then you can build an NPC that specifically targets the glass-cannons Achilles heel.
After a couple of sessions (probably a good half dozen...depending on how stubborn your players are), they'll be begging
to get rid of MC'ing or at least restrict it somehow. In a nutshell, I see munchkin'ing MC/Feat builds the same way I see use of poison in a campaign; most ignorant players are all for it in the beginning...until their ignorance is filled with the knowledge that what's good for the goose, is good for the gander....and the DM has a LOT more ganders!
Now I know I'll hear cries of "You are just a petty DM out to get your players and stomp on their fun!", so I'll just say this now. No, I'm not. It's my job as DM (and creator or my campaign world...upon which *I* have poured hours and hours of time, oodles of creativity, and, effectively, my heart and soul into it), to run a believable and fun campaign so that everyone enjoys it for the longest time possible. Players, IME, are often their own worst enemies. They may think allowing Option X, Y or Z will be the cats meow...until they see the consequences of their actions. What they 'want' is to feel cool and capable...and they mistakenly believe that adding options will give them this. It won't. A quick fix, sure, it will be 'awesome' for one or two sessions. Then the high wares off and they are looking for the next "fix". The only way to maintain this is to constantly add new options. This will
result in the implosion of a campaign. Why do you think everything after 2e has been focused around "Adventure Paths" and 'expected' advancement to level 20 in about a year or so of play time? Because WotC/Paizo have to sell more and more "options"...and as more are added to a campaign, the faster critical mass is reached. If a campaign 'ends' at level 20 after a year of play it doesn't 'implode'...thus, the mistaken belief that there is nothing wrong with adding more and more options.
I'm starting to go waaaay off on a tangent here, so I'll stop.
Suffice it to say, make sure you players actually know what they are asking for before you give it to them. If they still want to use MC, then say ok...and don't say you didn't warn them!
Paul L. Ming