Over 30 levels, a PC can expect to get a +15 to hit from leveling, a +4 from stat increases, and a +6 enhancement bonus. That adds up to +25 to hit vs first level. Over that same 30 levels, monster's defenses have gone up by 29. So, if you're hitting on a 10 at 1st level, on average, at 30th, you'll be hitting on a about a 14.
Powers, especially your leader's powers, may compensate for some of that. For instance, your warlord's Lead the Attack might have given you +4 to hit at 1st, level, but similar powers (and he'll have more of them) might give you a +8 at 30th.
When attacking non-AC defenses, you hit quite well if you pick the target's weaker defenses. Attacking the weaker of FORT or REF will get you a +1 or 2 or more, compared to attacking the stronger, and a +3 or more compared to attacking AC. If you can attack either FORT or REF or can attack WILL, you can generally hit just fine with a mere +3. With an 18 or 20, you can be less obsessive about deducing and attacking lower defenses.