It's interesting though. When we first started playing, solo's were scary and often quite grindy. Now, they play out as rather whimpy if they aren't a few levels above the party or teamed up with a couple of other monsters.
Could be the selection of monster, that the party is just getting really good at what they do or both. More damage and hit points are fine in my book when coupled with lower defenses.
Well, i just ran our first solo fight yesterday with five 5th level heroes and one goblin NPC against an 8th level solo. I designed the monster with MM2 tenets in mind, and tried to give it some cool control powers. It was a nasty lurker that more or less ambushed the party. Here's how it ended about 10 rounds later:
The wizard botched, and i mean REALLY botched his rolls and was more or less ineffective. The dwarf fighter used Pinning Strike fairly early on and for the rest of the encounter the spider could NOT move. That just slaughtered it. It had a web attack it used to good effect in round 1, but then the PCs closed in and the damage dealers started whacking at it. The only character who was ever in danger was the single warlord who was grabbed, squeezed and bitten by poison several times.
While we were playing this out i told the players how much damage they'd done, what the monsters attacks were, how it had 2 action points, and when they had it close to death. I wouldn't normally do that but i wanted them to see behind the scenes way solos worked.
It scared them plenty bad at first (as any huge demonic spider-thing of Torog would do) but ultimately the fight was a little lackluster for me, as i could tell after a certain point that this thing was just screwed and there was nothing i could do.
It had less hit points than a MM1 solo would have, and it had some interesting attacks that could potentially dish out lots of damage. It also had ranged web and a sedative spray (that it never used) and a cool adhesive lure that drags prey closer. There was also terrain features that added some tactical nuances to the encounter, like the dwarf almost getting pulled into a river with webbing.
My opinion is that the monster needed HELP. It needed allies. It needed flanking opportunities and it needed for someone to drag that goddamn dwarf away and ruin her Pinning Strike. Plus, the barbarain was knocking the monster prone every other round, but it couldn't run away anyhow.
It was still very fun though, and i look forward to running a solo again, but it takes some careful tweaking to make a solo fight as fun as an encounter with many enemies. And avoiding a grindfest where the PCs will inevitably just win by war of attrition.