People don't like the half-elf because multiclassing is turning out to be very, very hard in 4th edition. You can't just pick any random class and pull a power from it. First, you have to make sure that the power uses a stat that is high for you. Then, you have to make sure that the tool required for that power is compatible with the tools that you use. If those conditions are not both met, then you are going to find out that your multiclass skill has a terrible chance to hit and/or low damage and/or requires you to juggle weapons in combat.
At low levels, the 'tool' thing isn't necessarily as serious. For instance, a Tactical Warlord might pick up Scorching Burst (like the KOTS Warlord pregen) and find it to be quite handy. However, as the Warlord gains levels, they'll find that their to-hit lags significantly behind a 'real' wizard, unless he both invests in a magic wand, and spends actions swapping it in and out during combat.